diff --git a/src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/ClientConfigPaths.cs b/src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/ClientConfigPaths.cs
index a92612ca0b0f4..224b134a8a147 100644
--- a/src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/ClientConfigPaths.cs
+++ b/src/libraries/System.Configuration.ConfigurationManager/src/System/Configuration/ClientConfigPaths.cs
@@ -92,7 +92,23 @@ private ClientConfigPaths(string exePath, bool includeUserConfig)
                 }
             }
 
-            if (!string.IsNullOrEmpty(ApplicationUri))
+            string externalConfigPath = AppDomain.CurrentDomain.GetData("APP_CONFIG_FILE") as string;
+            if (!string.IsNullOrEmpty(externalConfigPath))
+            {
+                if (Uri.IsWellFormedUriString(externalConfigPath, UriKind.Absolute))
+                {
+                    Uri externalConfigUri = new Uri(externalConfigPath);
+                    if (externalConfigUri.IsFile)
+                    {
+                        ApplicationConfigUri = externalConfigUri.LocalPath;
+                    }
+                }
+                else
+                {
+                    ApplicationConfigUri = Path.GetFullPath(externalConfigPath);
+                }
+            }
+            else if (!string.IsNullOrEmpty(ApplicationUri))
             {
                 string applicationPath = ApplicationUri;
                 if (isSingleFile)
diff --git a/src/libraries/System.Configuration.ConfigurationManager/tests/System.Configuration.ConfigurationManager.Tests.csproj b/src/libraries/System.Configuration.ConfigurationManager/tests/System.Configuration.ConfigurationManager.Tests.csproj
index 94670bcb997df..816d4ca00292e 100644
--- a/src/libraries/System.Configuration.ConfigurationManager/tests/System.Configuration.ConfigurationManager.Tests.csproj
+++ b/src/libraries/System.Configuration.ConfigurationManager/tests/System.Configuration.ConfigurationManager.Tests.csproj
@@ -65,6 +65,7 @@
     <Compile Include="System\Configuration\ConfigurationElementCollectionTests.cs" />
     <Compile Include="System\Configuration\ConfigurationElementTests.cs" />
     <Compile Include="System\Configuration\ConfigurationPropertyAttributeTests.cs" />
+    <Compile Include="System\Configuration\ConfigurationPathTests.cs" />
     <Compile Include="System\Configuration\CustomHostTests.cs" />
     <Compile Include="System\Configuration\ConfigurationPropertyTests.cs" />
     <Compile Include="System\Configuration\ConfigurationTests.cs" />
diff --git a/src/libraries/System.Configuration.ConfigurationManager/tests/System/Configuration/ConfigurationPathTests.cs b/src/libraries/System.Configuration.ConfigurationManager/tests/System/Configuration/ConfigurationPathTests.cs
new file mode 100644
index 0000000000000..5d2442dffbfe0
--- /dev/null
+++ b/src/libraries/System.Configuration.ConfigurationManager/tests/System/Configuration/ConfigurationPathTests.cs
@@ -0,0 +1,92 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Configuration;
+using System.IO;
+using Microsoft.DotNet.RemoteExecutor;
+using Xunit;
+
+namespace System.ConfigurationTests
+{
+    public class ConfigurationPathTests
+    {
+        private const string ConfigName = "APP_CONFIG_FILE";
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void CustomAppConfigIsUsedWhenSpecifiedAsRelativePath()
+        {
+            const string SettingName = "test_CustomAppConfigIsUsedWhenSpecified";
+            string expectedSettingValue = Guid.NewGuid().ToString();
+            string configFilePath = CreateAppConfigFileWithSetting(SettingName, expectedSettingValue);
+
+            RemoteExecutor.Invoke((string configFilePath, string expectedSettingValue) => {
+                AppDomain.CurrentDomain.SetData(ConfigName, configFilePath);
+                Assert.Equal(expectedSettingValue, ConfigurationManager.AppSettings[SettingName]);
+            }, configFilePath, expectedSettingValue).Dispose();
+        }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void CustomAppConfigIsUsedWhenSpecifiedAsAbsolutePath()
+        {
+            const string SettingName = "test_CustomAppConfigIsUsedWhenSpecified";
+            string expectedSettingValue = Guid.NewGuid().ToString();
+            string configFilePath = Path.GetFullPath(CreateAppConfigFileWithSetting(SettingName, expectedSettingValue));
+
+            RemoteExecutor.Invoke((string configFilePath, string expectedSettingValue) => {
+                AppDomain.CurrentDomain.SetData(ConfigName, configFilePath);
+                Assert.Equal(expectedSettingValue, ConfigurationManager.AppSettings[SettingName]);
+            }, configFilePath, expectedSettingValue).Dispose();
+        }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void CustomAppConfigIsUsedWhenSpecifiedAsAbsoluteUri()
+        {
+            const string SettingName = "test_CustomAppConfigIsUsedWhenSpecified";
+            string expectedSettingValue = Guid.NewGuid().ToString();
+            string configFilePath = new Uri(Path.GetFullPath(CreateAppConfigFileWithSetting(SettingName, expectedSettingValue))).ToString();
+
+            RemoteExecutor.Invoke((string configFilePath, string expectedSettingValue) => {
+                AppDomain.CurrentDomain.SetData(ConfigName, configFilePath);
+                Assert.Equal(expectedSettingValue, ConfigurationManager.AppSettings[SettingName]);
+            }, configFilePath, expectedSettingValue).Dispose();
+        }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void NoErrorWhenCustomAppConfigIsSpecifiedAndItDoesNotExist()
+        {
+            RemoteExecutor.Invoke(() =>
+            {
+                AppDomain.CurrentDomain.SetData(ConfigName, "non-existing-file.config");
+                Assert.Null(ConfigurationManager.AppSettings["AnySetting"]);
+            }).Dispose();
+        }
+
+        [ConditionalFact(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        public void MalformedAppConfigCausesException()
+        {
+            const string SettingName = "AnySetting";
+
+            // Following will cause malformed config file
+            string configFilePath = CreateAppConfigFileWithSetting(SettingName, "\"");
+
+            RemoteExecutor.Invoke((string configFilePath) => {
+                AppDomain.CurrentDomain.SetData(ConfigName, configFilePath);
+                Assert.Throws<ConfigurationErrorsException>(() => ConfigurationManager.AppSettings[SettingName]);
+            }, configFilePath).Dispose();
+        }
+
+        private static string CreateAppConfigFileWithSetting(string key, string rawUnquotedValue)
+        {
+            string fileName = Path.GetRandomFileName() + ".config";
+            File.WriteAllText(fileName,
+                @$"<?xml version=""1.0"" encoding=""utf-8"" ?>
+<configuration>
+  <appSettings>
+    <add key=""{key}"" value=""{rawUnquotedValue}""/>
+  </appSettings>
+</configuration>");
+
+            return fileName;
+        }
+    }
+}