diff --git a/DESCRIPTION b/DESCRIPTION
index cb31b341..5a4297aa 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -18,7 +18,7 @@ Description: Gives the ability to automatically generate and serve an HTTP API
     from R functions using the annotations in the R documentation around your
     functions.
 Depends:
-    R (>= 3.0.0)
+    R (>= 3.5.0)
 Imports:
     R6 (>= 2.0.0),
     stringi (>= 0.3.0),
diff --git a/NEWS.md b/NEWS.md
index cd188a2d..a3b65a6c 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,19 @@
 # plumber (development version)
 
+## Breaking changes
+
+* When mounting a router at an existing location, the old mounted router is removed and the router to be mounted is added to the end of the mount list (default; `after = NULL`). This makes it so prior mount calls do not affect current mount calls. (#882)
+
+* The default value of `debug` has been changed from `base::interactive()` to `rlang::is_interactive()`. This allows for tests to be consistent when run interactively or in batch mode. (#882)
+
+## New features
+
+* Added support for `pr_mount(after=)` / `Plumber$mount(after=)` which allows for mounts to be added in non-sequential order (#882)
+
+* Always mount OpenAPI documentation at `/__docs__/` as the first mount to have preference when using conflicting mount locations (e.g. a static mount at `/`). (#882)
+
+## Bug fixes
+
 
 # plumber 1.2.1
 
diff --git a/R/plumber.R b/R/plumber.R
index 03cefd73..1620ef69 100644
--- a/R/plumber.R
+++ b/R/plumber.R
@@ -143,7 +143,7 @@ Plumber <- R6Class(
     #' Mac OS X, port numbers smaller than 1025 require root privileges.
     #'
     #' This value does not need to be explicitly assigned. To explicitly set it, see [options_plumber()].
-    #' @param debug If `TRUE`, it will provide more insight into your API errors. Using this value will only last for the duration of the run. If a `$setDebug()` has not been called, `debug` will default to `interactive()` at `$run()` time. See `$setDebug()` for more details.
+    #' @param debug If `TRUE`, it will provide more insight into your API errors. Using this value will only last for the duration of the run. If a `$setDebug()` has not been called, `debug` will default to [`rlang::is_interactive()`] at `$run()` time. See `$setDebug()` for more details.
     #' @param swagger Deprecated. Please use `docs` instead. See `$setDocs(docs)` or `$setApiSpec()` for more customization.
     #' @param swaggerCallback An optional single-argument function that is
     #'   called back with the URL to an OpenAPI user interface when one becomes
@@ -232,7 +232,7 @@ Plumber <- R6Class(
       }, add = TRUE)
       # Fix the debug value while running.
       self$setDebug(
-        # Order: Run method param, internally set value, is interactive()
+        # Order: Run method param, internally set value, `is_interactive()`
         # `$getDebug()` is dynamic given `setDebug()` has never been called.
         rlang::maybe_missing(debug, self$getDebug())
       )
@@ -252,7 +252,7 @@ Plumber <- R6Class(
       # Set and restore the wd to make it appear that the proc is running local to the file's definition.
       if (!is.null(private$filename)) {
         old_wd <- setwd(dirname(private$filename))
-        on.exit({setwd(old_wd)}, add = TRUE)
+        on.exit(setwd(old_wd), add = TRUE, after = FALSE)
       }
 
       if (isTRUE(docs_info$enabled)) {
@@ -264,10 +264,11 @@ Plumber <- R6Class(
           callback = swaggerCallback,
           quiet = quiet
         )
-        on.exit(unmount_docs(self, docs_info), add = TRUE)
+        on.exit(unmount_docs(self, docs_info), add = TRUE, after = FALSE)
       }
 
-      on.exit(private$runHooks("exit"), add = TRUE)
+      # Run user exit hooks given docs and working directory
+      on.exit(private$runHooks("exit"), add = TRUE, after = FALSE)
 
       httpuv::runServer(host, port, self)
     },
@@ -278,8 +279,13 @@ Plumber <- R6Class(
     #' by paths which is a great technique for decomposing large APIs into smaller files.
     #'
     #' See also: [pr_mount()]
-    #' @param path a character string. Where to mount router.
-    #' @param router a Plumber router. Router to be mounted.
+    #' @param path a character string. Where to mount the sub router.
+    #' @param router a Plumber router. Sub router to be mounted.
+    #' @param ... Ignored. Used for possible parameter expansion.
+    #' @param after If `NULL` (default), the router will be appended to the end
+    #'   of the mounts. If a number, the router will be inserted at the given
+    #'   index. E.g. `after = 0` will prepend the sub router, giving it
+    #'   preference over other mounted routers.
     #' @examples
     #' \dontrun{
     #' root <- pr()
@@ -290,7 +296,9 @@ Plumber <- R6Class(
     #' products <- Plumber$new("products.R")
     #' root$mount("/products", products)
     #' }
-    mount = function(path, router) {
+    mount = function(path, router, ..., after = NULL) {
+      ellipsis::check_dots_empty()
+
       # Ensure that the path has both a leading and trailing slash.
       if (!grepl("^/", path)) {
         path <- paste0("/", path)
@@ -299,7 +307,15 @@ Plumber <- R6Class(
         path <- paste0(path, "/")
       }
 
-      private$mnts[[path]] <- router
+      # Remove prior mount if it exists
+      self$unmount(path)
+
+      # Add new mount
+      # Mount order matters
+      after <- after %||% length(private$mnts)
+      mntList <- list()
+      mntList[[path]] <- router
+      private$mnts <- append(private$mnts, mntList, after = after)
     },
     #' @description Unmount a Plumber router
     #' @param path a character string. Where to unmount router.
@@ -963,11 +979,11 @@ Plumber <- R6Class(
     #'
     #' See also: `$getDebug()` and [pr_set_debug()]
     #' @param debug `TRUE` provides more insight into your API errors.
-    setDebug = function(debug = interactive()) {
+    setDebug = function(debug = is_interactive()) {
       stopifnot(length(debug) == 1)
       private$debug <- isTRUE(debug)
     },
-    #' @description Retrieve the `debug` value. If it has never been set, the result of `interactive()` will be used.
+    #' @description Retrieve the `debug` value. If it has never been set, the result of [`rlang::is_interactive()`] will be used.
     #'
     #' See also: `$getDebug()` and [pr_set_debug()]
     getDebug = function() {
@@ -1335,8 +1351,10 @@ upgrade_docs_parameter <- function(docs, ...) {
 
 
 
+#' @importFrom rlang is_interactive
+# Method needed for testing mocking
 default_debug <- function() {
-  interactive()
+  is_interactive()
 }
 
 
diff --git a/R/pr.R b/R/pr.R
index 40f1b4be..9237054d 100644
--- a/R/pr.R
+++ b/R/pr.R
@@ -210,8 +210,13 @@ pr_head <- function(pr,
 #' returns the updated router.
 #'
 #' @param pr The host Plumber router.
-#' @param path A character string. Where to mount router.
-#' @param router A Plumber router. Router to be mounted.
+#' @param path a character string. Where to mount the sub router.
+#' @param router a Plumber router. Sub router to be mounted.
+#' @param ... Ignored. Used for possible parameter expansion.
+#' @param after If `NULL` (default), the router will be appended to the end
+#'   of the mounts. If a number, the router will be inserted at the given
+#'   index. E.g. `after = 0` will prepend the sub router, giving it
+#'   preference over other mounted routers.
 #'
 #' @return A Plumber router with the supplied router mounted
 #'
@@ -229,9 +234,11 @@ pr_head <- function(pr,
 #' @export
 pr_mount <- function(pr,
                      path,
-                     router) {
+                     router,
+                     ...,
+                     after = NULL) {
   validate_pr(pr)
-  pr$mount(path = path, router = router)
+  pr$mount(path = path, router = router, ..., after = after)
   pr
 }
 
@@ -490,7 +497,7 @@ pr_filter <- function(pr,
 #' @param ... Should be empty.
 #' @param debug If `TRUE`, it will provide more insight into your API errors.
 #'   Using this value will only last for the duration of the run.
-#'   If [pr_set_debug()] has not been called, `debug` will default to `interactive()` at [pr_run()] time
+#'   If [pr_set_debug()] has not been called, `debug` will default to [`rlang::is_interactive()`] at [pr_run()] time
 #' @param docs Visual documentation value to use while running the API.
 #'   This value will only be used while running the router.
 #'   If missing, defaults to information previously set with [pr_set_docs()].
diff --git a/R/pr_set.R b/R/pr_set.R
index dd66561f..375563fb 100644
--- a/R/pr_set.R
+++ b/R/pr_set.R
@@ -98,7 +98,7 @@ pr_set_error <- function(pr, fun) {
 #' Set debug value to include error messages of routes cause an error
 #'
 #' To hide any error messages in production, set the debug value to `FALSE`.
-#' The `debug` value is enabled by default for [interactive()] sessions.
+#' The `debug` value is enabled by default for [`rlang::is_interactive()`] sessions.
 #'
 #' @template param_pr
 #' @param debug `TRUE` provides more insight into your API errors.
@@ -118,7 +118,7 @@ pr_set_error <- function(pr, fun) {
 #'   pr_get("/boom", function() stop("boom")) %>%
 #'   pr_run()
 #' }
-pr_set_debug <- function(pr, debug = interactive()) {
+pr_set_debug <- function(pr, debug = is_interactive()) {
   validate_pr(pr)
   pr$setDebug(debug = debug)
   pr
diff --git a/R/ui.R b/R/ui.R
index f6bcf11e..5708b51e 100644
--- a/R/ui.R
+++ b/R/ui.R
@@ -1,4 +1,5 @@
 #' @include globals.R
+docs_root <- paste0("/__docs__/")
 
 # Mount OpenAPI and Docs
 #' @noRd
@@ -184,7 +185,6 @@ register_docs <- function(name, index, static = NULL) {
   stopifnot(is.function(index))
   if (!is.null(static)) stopifnot(is.function(static))
 
-  docs_root <- paste0("/__docs__/")
   docs_paths <- c("/index.html", "/")
   mount_docs_func <- function(pr, api_url, ...) {
     # Save initial extra argument values
@@ -211,7 +211,7 @@ register_docs <- function(name, index, static = NULL) {
       message("")
     }
 
-    pr$mount(docs_root, docs_router)
+    pr$mount(docs_root, docs_router, after = 0)
 
     # add legacy swagger redirects (RStudio Connect)
     redirect_info <- swagger_redirects()
diff --git a/man/Plumber.Rd b/man/Plumber.Rd
index 576d47c8..1c8b0cf2 100644
--- a/man/Plumber.Rd
+++ b/man/Plumber.Rd
@@ -273,15 +273,22 @@ by paths which is a great technique for decomposing large APIs into smaller file
 
 See also: \code{\link[=pr_mount]{pr_mount()}}
 \subsection{Usage}{
-\if{html}{\out{<div class="r">}}\preformatted{Plumber$mount(path, router)}\if{html}{\out{</div>}}
+\if{html}{\out{<div class="r">}}\preformatted{Plumber$mount(path, router, ..., after = NULL)}\if{html}{\out{</div>}}
 }
 
 \subsection{Arguments}{
 \if{html}{\out{<div class="arguments">}}
 \describe{
-\item{\code{path}}{a character string. Where to mount router.}
+\item{\code{path}}{a character string. Where to mount the sub router.}
 
-\item{\code{router}}{a Plumber router. Router to be mounted.}
+\item{\code{router}}{a Plumber router. Sub router to be mounted.}
+
+\item{\code{...}}{Ignored. Used for possible parameter expansion.}
+
+\item{\code{after}}{If \code{NULL} (default), the router will be appended to the end
+of the mounts. If a number, the router will be inserted at the given
+index. E.g. \code{after = 0} will prepend the sub router, giving it
+preference over other mounted routers.}
 }
 \if{html}{\out{</div>}}
 }
diff --git a/man/pr_mount.Rd b/man/pr_mount.Rd
index 7f08d1c1..e6a07e19 100644
--- a/man/pr_mount.Rd
+++ b/man/pr_mount.Rd
@@ -4,14 +4,21 @@
 \alias{pr_mount}
 \title{Mount a Plumber router}
 \usage{
-pr_mount(pr, path, router)
+pr_mount(pr, path, router, ..., after = NULL)
 }
 \arguments{
 \item{pr}{The host Plumber router.}
 
-\item{path}{A character string. Where to mount router.}
+\item{path}{a character string. Where to mount the sub router.}
 
-\item{router}{A Plumber router. Router to be mounted.}
+\item{router}{a Plumber router. Sub router to be mounted.}
+
+\item{...}{Ignored. Used for possible parameter expansion.}
+
+\item{after}{If \code{NULL} (default), the router will be appended to the end
+of the mounts. If a number, the router will be inserted at the given
+index. E.g. \code{after = 0} will prepend the sub router, giving it
+preference over other mounted routers.}
 }
 \value{
 A Plumber router with the supplied router mounted
diff --git a/tests/testthat/files/router.R b/tests/testthat/files/router.R
index a52a543f..2f68ae0e 100644
--- a/tests/testthat/files/router.R
+++ b/tests/testthat/files/router.R
@@ -61,3 +61,21 @@ function(res){
 function(){
   "dual path"
 }
+
+#* @plumber
+function(pr) {
+  mnt_1 <-
+    pr() %>%
+    pr_get("/hello", function() "say hello")
+  mnt_2 <-
+    pr() %>%
+    pr_get("/world", function() "say hello world")
+  mnt_0 <-
+    pr() %>%
+    pr_get("/world", function() "say hello world")
+
+  pr %>%
+    pr_mount("/say", mnt_1) %>%
+    pr_mount("/say/hello", mnt_2) %>%
+    pr_mount("/first", mnt_0, after = 0)
+}
diff --git a/tests/testthat/helper-interrupt.R b/tests/testthat/helper-interrupt.R
new file mode 100644
index 00000000..74f7ceeb
--- /dev/null
+++ b/tests/testthat/helper-interrupt.R
@@ -0,0 +1,6 @@
+
+with_interrupt <- function(expr, delay = 0) {
+  # Causes pr_run() to immediately exit
+  later::later(httpuv::interrupt, delay = delay)
+  force(expr)
+}
diff --git a/tests/testthat/test-plumber-run.R b/tests/testthat/test-plumber-run.R
index ffdf829b..b70c5929 100644
--- a/tests/testthat/test-plumber-run.R
+++ b/tests/testthat/test-plumber-run.R
@@ -1,10 +1,4 @@
 
-with_interrupt <- function(expr) {
-  # Causes pr_run() to immediately exit
-  later::later(httpuv::interrupt)
-  force(expr)
-}
-
 test_that("quiet=TRUE suppresses startup messages", {
   with_interrupt({
     expect_message(pr() %>% pr_run(quiet = TRUE), NA)
diff --git a/tests/testthat/test-routing.R b/tests/testthat/test-routing.R
index 224d0845..42daa29f 100644
--- a/tests/testthat/test-routing.R
+++ b/tests/testthat/test-routing.R
@@ -20,6 +20,15 @@ test_that("Routing to errors and 404s works", {
   expect_equal(r$route(make_req("GET", "/path1"), res), "dual path")
   expect_equal(r$route(make_req("GET", "/path2"), res), "dual path")
 
+  ### Won't work yet
+  ## Mounts fall back to parent router when route is not found
+  # # Mount at `/say` with route `/hello`
+  # expect_equal(r$route(make_req("GET", "/say/hello"), res), "say hello")
+  # # Mount at `/say/hello` with route `/world`
+  # expect_equal(r$route(make_req("GET", "/say/hello/world"), res), "say hello world")
+
+  expect_equal(names(r$mounts), c("/first/", "/say/", "/say/hello/"))
+
   expect_equal(errors, 0)
   expect_equal(notFounds, 0)
 
diff --git a/tests/testthat/test-ui.R b/tests/testthat/test-ui.R
new file mode 100644
index 00000000..a166111e
--- /dev/null
+++ b/tests/testthat/test-ui.R
@@ -0,0 +1,26 @@
+
+test_that("doc mounts are prepended", {
+
+  assertions <- 0
+
+  root <-
+    test_path("files/static.R") %>%
+    pr() %>%
+    pr_set_docs(TRUE) %>%
+    pr_hook("exit", function(...) {
+      expect_length(root$mounts, 3)
+      assertions <<- assertions + 1
+      expect_equal(names(root$mounts)[1], "/__docs__/")
+      assertions <<- assertions + 1
+    })
+
+  expect_length(root$mounts, 2)
+  assertions <- assertions + 1
+
+  # no docs. do not find printed message
+  with_interrupt({
+    root %>% pr_run()
+  })
+
+  expect_equal(assertions, 3)
+})