diff --git a/main/api/src/mill/api/Logger.scala b/main/api/src/mill/api/Logger.scala
index 3c6967558e9..c9053d158bf 100644
--- a/main/api/src/mill/api/Logger.scala
+++ b/main/api/src/mill/api/Logger.scala
@@ -2,6 +2,8 @@ package mill.api
 
 import java.io.{InputStream, PrintStream}
 
+import mill.main.client.lock.{Lock, Locked}
+
 /**
  * The standard logging interface of the Mill build tool.
  *
@@ -53,4 +55,14 @@ trait Logger {
   def debugEnabled: Boolean = false
 
   def close(): Unit = ()
+
+  def waitForLock(lock: Lock): Locked = {
+    val tryLocked = lock.tryLock()
+    if (tryLocked.isLocked())
+      tryLocked
+    else {
+      info("Waiting for lock...")
+      lock.lock()
+    }
+  }
 }
diff --git a/main/client/src/mill/main/client/OutFiles.java b/main/client/src/mill/main/client/OutFiles.java
index 872560cfda3..14595a7d699 100644
--- a/main/client/src/mill/main/client/OutFiles.java
+++ b/main/client/src/mill/main/client/OutFiles.java
@@ -53,4 +53,9 @@ public class OutFiles {
      */
     final public static String millNoServer = "mill-no-server";
 
+    /**
+     * Lock file used for exclusive access to the Mill output directory
+     */
+    final public static String millLock = "mill-lock";
+
 }
diff --git a/main/client/src/mill/main/client/lock/Lock.java b/main/client/src/mill/main/client/lock/Lock.java
index 6d729c0ebd6..1169ccff5b6 100644
--- a/main/client/src/mill/main/client/lock/Lock.java
+++ b/main/client/src/mill/main/client/lock/Lock.java
@@ -15,4 +15,13 @@ public void await() throws Exception {
      */
     public abstract boolean probe() throws Exception;
     public void delete() throws Exception {}
+
+    public static Lock file(String path) throws Exception {
+        return new FileLock(path);
+    }
+
+    public static Lock memory() {
+        return new MemoryLock();
+    }
+
 }
diff --git a/main/eval/src/mill/eval/Evaluator.scala b/main/eval/src/mill/eval/Evaluator.scala
index 1a1d2853cb5..5258d39c456 100644
--- a/main/eval/src/mill/eval/Evaluator.scala
+++ b/main/eval/src/mill/eval/Evaluator.scala
@@ -4,6 +4,7 @@ import mill.api.{CompileProblemReporter, DummyTestReporter, Result, TestReporter
 import mill.api.Strict.Agg
 import mill.define.{BaseModule, Segments, Task}
 import mill.eval.Evaluator.{Results, formatFailing}
+import mill.main.client.lock.Lock
 import mill.util.{ColorLogger, MultiBiMap}
 
 import scala.annotation.nowarn
@@ -19,6 +20,7 @@ trait Evaluator {
   def rootModule: BaseModule
   def effectiveThreadCount: Int
   def outPath: os.Path
+  def outPathLock: Lock
   def externalOutPath: os.Path
   def pathsResolver: EvaluatorPathsResolver
   def workerCache: collection.Map[Segments, (Int, Val)]
diff --git a/main/eval/src/mill/eval/EvaluatorCore.scala b/main/eval/src/mill/eval/EvaluatorCore.scala
index 777640d7eba..92364f6c4ee 100644
--- a/main/eval/src/mill/eval/EvaluatorCore.scala
+++ b/main/eval/src/mill/eval/EvaluatorCore.scala
@@ -12,6 +12,7 @@ import mill.util._
 import java.util.concurrent.atomic.{AtomicBoolean, AtomicInteger}
 import scala.collection.mutable
 import scala.concurrent._
+import scala.util.Using
 
 /**
  * Core logic of evaluating tasks, without any user-facing helper methods
@@ -32,8 +33,6 @@ private[mill] trait EvaluatorCore extends GroupEvaluator {
       logger: ColorLogger = baseLogger,
       serialCommandExec: Boolean = false
   ): Evaluator.Results = {
-    os.makeDir.all(outPath)
-
     PathRef.validatedPaths.withValue(new PathRef.ValidatedPaths()) {
       val ec =
         if (effectiveThreadCount == 1) ExecutionContexts.RunNow
@@ -43,7 +42,19 @@ private[mill] trait EvaluatorCore extends GroupEvaluator {
         if (effectiveThreadCount == 1) ""
         else s"#${if (effectiveThreadCount > 9) f"$threadId%02d" else threadId} "
 
-      try evaluate0(goals, logger, reporter, testReporter, ec, contextLoggerMsg, serialCommandExec)
+      try
+        Using.resource(logger.waitForLock(outPathLock)) { _ =>
+          os.makeDir.all(outPath)
+          evaluate0(
+            goals,
+            logger,
+            reporter,
+            testReporter,
+            ec,
+            contextLoggerMsg,
+            serialCommandExec
+          )
+        }
       finally ec.close()
     }
   }
diff --git a/main/eval/src/mill/eval/EvaluatorImpl.scala b/main/eval/src/mill/eval/EvaluatorImpl.scala
index 508bf7b1f5c..d6b63870c57 100644
--- a/main/eval/src/mill/eval/EvaluatorImpl.scala
+++ b/main/eval/src/mill/eval/EvaluatorImpl.scala
@@ -3,6 +3,7 @@ package mill.eval
 import mill.api.{CompileProblemReporter, Strict, TestReporter, Val}
 import mill.api.Strict.Agg
 import mill.define._
+import mill.main.client.lock.Lock
 import mill.util._
 
 import scala.collection.mutable
@@ -17,6 +18,7 @@ private[mill] case class EvaluatorImpl(
     home: os.Path,
     workspace: os.Path,
     outPath: os.Path,
+    outPathLock: Lock,
     externalOutPath: os.Path,
     override val rootModule: mill.define.BaseModule,
     baseLogger: ColorLogger,
diff --git a/main/eval/src/mill/eval/GroupEvaluator.scala b/main/eval/src/mill/eval/GroupEvaluator.scala
index 7d1529801ea..410a5742385 100644
--- a/main/eval/src/mill/eval/GroupEvaluator.scala
+++ b/main/eval/src/mill/eval/GroupEvaluator.scala
@@ -5,6 +5,7 @@ import mill.api.Strict.Agg
 import mill.api._
 import mill.define._
 import mill.eval.Evaluator.TaskResult
+import mill.main.client.lock.Lock
 import mill.util._
 
 import java.io.PrintStream
@@ -23,6 +24,7 @@ private[mill] trait GroupEvaluator {
   def home: os.Path
   def workspace: os.Path
   def outPath: os.Path
+  def outPathLock: Lock
   def externalOutPath: os.Path
   def rootModule: mill.define.BaseModule
   def classLoaderSigHash: Int
diff --git a/runner/src/mill/runner/MillBuildBootstrap.scala b/runner/src/mill/runner/MillBuildBootstrap.scala
index ad84d5eb956..b2969fb3836 100644
--- a/runner/src/mill/runner/MillBuildBootstrap.scala
+++ b/runner/src/mill/runner/MillBuildBootstrap.scala
@@ -6,12 +6,15 @@ import mill.main.client.CodeGenConstants._
 import mill.api.{PathRef, Val, internal}
 import mill.eval.Evaluator
 import mill.main.RunScript
+import mill.main.client.lock.Lock
 import mill.resolve.SelectMode
 import mill.define.{BaseModule, Discover, Segments}
 import mill.main.client.OutFiles.{millBuild, millRunnerState}
 
 import java.net.URLClassLoader
 
+import scala.util.Using
+
 /**
  * Logic around bootstrapping Mill, creating a [[MillBuildRootModule.BootstrapModule]]
  * and compiling builds/meta-builds and classloading their [[RootModule]]s so we
@@ -31,6 +34,7 @@ import java.net.URLClassLoader
 class MillBuildBootstrap(
     projectRoot: os.Path,
     output: os.Path,
+    outputLock: Lock,
     home: os.Path,
     keepGoing: Boolean,
     imports: Seq[String],
@@ -53,12 +57,14 @@ class MillBuildBootstrap(
   def evaluate(): Watching.Result[RunnerState] = CliImports.withValue(imports) {
     val runnerState = evaluateRec(0)
 
-    for ((frame, depth) <- runnerState.frames.zipWithIndex) {
-      os.write.over(
-        recOut(output, depth) / millRunnerState,
-        upickle.default.write(frame.loggedData, indent = 4),
-        createFolders = true
-      )
+    Using.resource(logger.waitForLock(outputLock)) { _ =>
+      for ((frame, depth) <- runnerState.frames.zipWithIndex) {
+        os.write.over(
+          recOut(output, depth) / millRunnerState,
+          upickle.default.write(frame.loggedData, indent = 4),
+          createFolders = true
+        )
+      }
     }
 
     Watching.Result(
@@ -344,6 +350,7 @@ class MillBuildBootstrap(
       home,
       projectRoot,
       recOut(output, depth),
+      outputLock,
       recOut(output, depth),
       rootModule,
       PrefixLogger(logger, "", tickerContext = bootLogPrefix),
diff --git a/runner/src/mill/runner/MillMain.scala b/runner/src/mill/runner/MillMain.scala
index e48179b2702..3e23517f8f3 100644
--- a/runner/src/mill/runner/MillMain.scala
+++ b/runner/src/mill/runner/MillMain.scala
@@ -9,6 +9,7 @@ import mill.api.{MillException, SystemStreams, WorkspaceRoot, internal}
 import mill.bsp.{BspContext, BspServerResult}
 import mill.main.BuildInfo
 import mill.main.client.OutFiles
+import mill.main.client.lock.Lock
 import mill.util.PrintLogger
 
 import java.lang.reflect.InvocationTargetException
@@ -215,6 +216,8 @@ object MillMain {
                     .map(_ => Seq(bspCmd))
                     .getOrElse(config.leftoverArgs.value.toList)
 
+                val out = os.Path(OutFiles.out, WorkspaceRoot.workspaceRoot)
+                val outLock = Lock.file((out / OutFiles.millLock).toString)
                 var repeatForBsp = true
                 var loopRes: (Boolean, RunnerState) = (false, RunnerState.empty)
                 while (repeatForBsp) {
@@ -231,7 +234,8 @@ object MillMain {
 
                       new MillBuildBootstrap(
                         projectRoot = WorkspaceRoot.workspaceRoot,
-                        output = os.Path(OutFiles.out, WorkspaceRoot.workspaceRoot),
+                        output = out,
+                        outputLock = outLock,
                         home = config.home,
                         keepGoing = config.keepGoing.value,
                         imports = config.imports,
diff --git a/testkit/src/mill/testkit/UnitTester.scala b/testkit/src/mill/testkit/UnitTester.scala
index 1d921c67477..411e7728676 100644
--- a/testkit/src/mill/testkit/UnitTester.scala
+++ b/testkit/src/mill/testkit/UnitTester.scala
@@ -5,6 +5,7 @@ import mill.api.Result.OuterStack
 import mill.api.{DummyInputStream, Result, SystemStreams, Val}
 import mill.define.{InputImpl, TargetImpl}
 import mill.eval.{Evaluator, EvaluatorImpl}
+import mill.main.client.lock.Lock
 import mill.resolve.{Resolve, SelectMode}
 import mill.util.PrintLogger
 import mill.api.Strict.Agg
@@ -55,7 +56,8 @@ class UnitTester(
     env: Map[String, String],
     resetSourcePath: Boolean
 )(implicit fullName: sourcecode.FullName) extends AutoCloseable {
-  val outPath: os.Path = module.millSourcePath / "out"
+  val outPath = module.millSourcePath / "out"
+  val outPathLock = Lock.file((module.millSourcePath / "out/.mill-lock").toString)
 
   if (resetSourcePath) {
     os.remove.all(module.millSourcePath)
@@ -90,6 +92,7 @@ class UnitTester(
     mill.api.Ctx.defaultHome,
     module.millSourcePath,
     outPath,
+    outPathLock,
     outPath,
     module,
     logger,