diff --git a/src/Runner.Common/HttpClientHandlerFactory.cs b/src/Runner.Common/HttpClientHandlerFactory.cs
index 4e0a88db718..badadb9690d 100644
--- a/src/Runner.Common/HttpClientHandlerFactory.cs
+++ b/src/Runner.Common/HttpClientHandlerFactory.cs
@@ -1,5 +1,7 @@
 using System;
+using System.IO;
 using System.Net.Http;
+using System.Security.Cryptography.X509Certificates;
 using GitHub.Runner.Sdk;
 
 namespace GitHub.Runner.Common
@@ -21,7 +23,60 @@ public HttpClientHandler CreateClientHandler(RunnerWebProxy webProxy)
                 client.ServerCertificateCustomValidationCallback = HttpClientHandler.DangerousAcceptAnyServerCertificateValidator;
             }
 
+            // Configure mTLS client certificate for proxy authentication
+            if (!string.IsNullOrEmpty(webProxy.HttpsProxyClientCert))
+            {
+                var clientCert = LoadClientCertificate(
+                    webProxy.HttpsProxyClientCert,
+                    webProxy.HttpsProxyClientKey);
+                if (clientCert != null)
+                {
+                    client.ClientCertificates.Add(clientCert);
+                }
+            }
+
             return client;
         }
+
+        private X509Certificate2 LoadClientCertificate(string certPath, string keyPath)
+        {
+            try
+            {
+                if (!File.Exists(certPath))
+                {
+                    Trace.Warning($"Client certificate file not found: {certPath}");
+                    return null;
+                }
+
+                // If key path is provided separately, load cert and key from separate files
+                if (!string.IsNullOrEmpty(keyPath))
+                {
+                    if (!File.Exists(keyPath))
+                    {
+                        Trace.Warning($"Client key file not found: {keyPath}");
+                        return null;
+                    }
+
+                    // Load certificate and private key from separate PEM files
+                    var certPem = File.ReadAllText(certPath);
+                    var keyPem = File.ReadAllText(keyPath);
+                    var cert = X509Certificate2.CreateFromPem(certPem, keyPem);
+
+                    // On Windows, we need to export and re-import to make the certificate usable
+                    // with SslStream/HttpClient
+                    return new X509Certificate2(cert.Export(X509ContentType.Pfx));
+                }
+                else
+                {
+                    // Assume the cert file contains both certificate and key (PFX/PKCS12 format)
+                    return new X509Certificate2(certPath);
+                }
+            }
+            catch (Exception ex)
+            {
+                Trace.Warning($"Failed to load client certificate: {ex.Message}");
+                return null;
+            }
+        }
     }
 }
diff --git a/src/Runner.Sdk/RunnerWebProxy.cs b/src/Runner.Sdk/RunnerWebProxy.cs
index 4c3c92a5502..a0505daaefd 100644
--- a/src/Runner.Sdk/RunnerWebProxy.cs
+++ b/src/Runner.Sdk/RunnerWebProxy.cs
@@ -22,6 +22,9 @@ public class RunnerWebProxy : IWebProxy
         private string _httpsProxyUsername;
         private string _httpsProxyPassword;
         private string _noProxyString;
+        private string _httpsProxyClientCert;
+        private string _httpsProxyClientKey;
+        private string _httpsProxyCACert;
 
         private readonly List<ByPassInfo> _noProxyList = new();
         private readonly HashSet<string> _noProxyUnique = new(StringComparer.OrdinalIgnoreCase);
@@ -35,6 +38,9 @@ public class RunnerWebProxy : IWebProxy
         public string HttpsProxyUsername => _httpsProxyUsername;
         public string HttpsProxyPassword => _httpsProxyPassword;
         public string NoProxyString => _noProxyString;
+        public string HttpsProxyClientCert => _httpsProxyClientCert;
+        public string HttpsProxyClientKey => _httpsProxyClientKey;
+        public string HttpsProxyCACert => _httpsProxyCACert;
 
         public List<ByPassInfo> NoProxyList => _noProxyList;
 
@@ -137,6 +143,37 @@ public RunnerWebProxy()
                 }
             }
 
+            // Load mTLS client certificate configuration for proxy connections
+            var httpsProxyClientCert = Environment.GetEnvironmentVariable("HTTPS_PROXY_CLIENT_CERT");
+            if (string.IsNullOrEmpty(httpsProxyClientCert))
+            {
+                httpsProxyClientCert = Environment.GetEnvironmentVariable("https_proxy_client_cert");
+            }
+            if (!string.IsNullOrEmpty(httpsProxyClientCert))
+            {
+                _httpsProxyClientCert = httpsProxyClientCert.Trim();
+            }
+
+            var httpsProxyClientKey = Environment.GetEnvironmentVariable("HTTPS_PROXY_CLIENT_KEY");
+            if (string.IsNullOrEmpty(httpsProxyClientKey))
+            {
+                httpsProxyClientKey = Environment.GetEnvironmentVariable("https_proxy_client_key");
+            }
+            if (!string.IsNullOrEmpty(httpsProxyClientKey))
+            {
+                _httpsProxyClientKey = httpsProxyClientKey.Trim();
+            }
+
+            var httpsProxyCACert = Environment.GetEnvironmentVariable("HTTPS_PROXY_CA_CERT");
+            if (string.IsNullOrEmpty(httpsProxyCACert))
+            {
+                httpsProxyCACert = Environment.GetEnvironmentVariable("https_proxy_ca_cert");
+            }
+            if (!string.IsNullOrEmpty(httpsProxyCACert))
+            {
+                _httpsProxyCACert = httpsProxyCACert.Trim();
+            }
+
             if (!string.IsNullOrEmpty(noProxyList))
             {
                 _noProxyString = noProxyList;
diff --git a/src/Test/L0/RunnerWebProxyL0.cs b/src/Test/L0/RunnerWebProxyL0.cs
index 5e339e0a32f..1f9eca70799 100644
--- a/src/Test/L0/RunnerWebProxyL0.cs
+++ b/src/Test/L0/RunnerWebProxyL0.cs
@@ -581,6 +581,52 @@ public void WebProxyFromEnvironmentVariablesWithPort80()
             }
         }
 
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void WebProxyClientCertificateFromEnvironmentVariables()
+        {
+            try
+            {
+                Environment.SetEnvironmentVariable("https_proxy", "http://127.0.0.1:9999");
+                Environment.SetEnvironmentVariable("HTTPS_PROXY_CLIENT_CERT", "/path/to/client.crt");
+                Environment.SetEnvironmentVariable("HTTPS_PROXY_CLIENT_KEY", "/path/to/client.key");
+                Environment.SetEnvironmentVariable("HTTPS_PROXY_CA_CERT", "/path/to/ca.crt");
+                var proxy = new RunnerWebProxy();
+
+                Assert.Equal("/path/to/client.crt", proxy.HttpsProxyClientCert);
+                Assert.Equal("/path/to/client.key", proxy.HttpsProxyClientKey);
+                Assert.Equal("/path/to/ca.crt", proxy.HttpsProxyCACert);
+            }
+            finally
+            {
+                CleanProxyEnv();
+            }
+        }
+
+        [Fact]
+        [Trait("Level", "L0")]
+        [Trait("Category", "Common")]
+        public void WebProxyClientCertificateLowerCaseFromEnvironmentVariables()
+        {
+            try
+            {
+                Environment.SetEnvironmentVariable("https_proxy", "http://127.0.0.1:9999");
+                Environment.SetEnvironmentVariable("https_proxy_client_cert", "/path/to/client.crt");
+                Environment.SetEnvironmentVariable("https_proxy_client_key", "/path/to/client.key");
+                Environment.SetEnvironmentVariable("https_proxy_ca_cert", "/path/to/ca.crt");
+                var proxy = new RunnerWebProxy();
+
+                Assert.Equal("/path/to/client.crt", proxy.HttpsProxyClientCert);
+                Assert.Equal("/path/to/client.key", proxy.HttpsProxyClientKey);
+                Assert.Equal("/path/to/ca.crt", proxy.HttpsProxyCACert);
+            }
+            finally
+            {
+                CleanProxyEnv();
+            }
+        }
+
         private void CleanProxyEnv()
         {
             Environment.SetEnvironmentVariable("http_proxy", null);
@@ -589,6 +635,12 @@ private void CleanProxyEnv()
             Environment.SetEnvironmentVariable("HTTPS_PROXY", null);
             Environment.SetEnvironmentVariable("no_proxy", null);
             Environment.SetEnvironmentVariable("NO_PROXY", null);
+            Environment.SetEnvironmentVariable("HTTPS_PROXY_CLIENT_CERT", null);
+            Environment.SetEnvironmentVariable("HTTPS_PROXY_CLIENT_KEY", null);
+            Environment.SetEnvironmentVariable("HTTPS_PROXY_CA_CERT", null);
+            Environment.SetEnvironmentVariable("https_proxy_client_cert", null);
+            Environment.SetEnvironmentVariable("https_proxy_client_key", null);
+            Environment.SetEnvironmentVariable("https_proxy_ca_cert", null);
         }
     }
 }