diff --git a/dlls/msvcr100/msvcr100.spec b/dlls/msvcr100/msvcr100.spec
index 307d00e11e5..f4baf442b53 100644
--- a/dlls/msvcr100/msvcr100.spec
+++ b/dlls/msvcr100/msvcr100.spec
@@ -1094,7 +1094,7 @@
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
 @ extern _mbctype MSVCRT_mbctype
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat_s(ptr long str)
diff --git a/dlls/msvcr110/msvcr110.spec b/dlls/msvcr110/msvcr110.spec
index baf10ae48a4..e9bdf422024 100644
--- a/dlls/msvcr110/msvcr110.spec
+++ b/dlls/msvcr110/msvcr110.spec
@@ -1451,7 +1451,7 @@
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
 @ extern _mbctype MSVCRT_mbctype
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat_s(ptr long str)
diff --git a/dlls/msvcr120/msvcr120.spec b/dlls/msvcr120/msvcr120.spec
index 8fa160170d5..13dacc15744 100644
--- a/dlls/msvcr120/msvcr120.spec
+++ b/dlls/msvcr120/msvcr120.spec
@@ -1462,7 +1462,7 @@
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
 @ extern _mbctype MSVCRT_mbctype
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat_s(ptr long str)
diff --git a/dlls/msvcr120_app/msvcr120_app.spec b/dlls/msvcr120_app/msvcr120_app.spec
index 2b2949cdc66..561c3774260 100644
--- a/dlls/msvcr120_app/msvcr120_app.spec
+++ b/dlls/msvcr120_app/msvcr120_app.spec
@@ -1302,7 +1302,7 @@
 @ cdecl _malloc_crt(long) msvcr120._malloc_crt
 # extern _mbcasemap
 @ extern _mbctype msvcr120._mbctype
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr) msvcr120._mblen_l
 @ cdecl _mbstowcs_l(ptr str long ptr) msvcr120._mbstowcs_l
 @ cdecl _mbstowcs_s_l(ptr ptr long str long ptr) msvcr120._mbstowcs_s_l
 @ cdecl _mbstrlen(str) msvcr120._mbstrlen
diff --git a/dlls/msvcr80/msvcr80.spec b/dlls/msvcr80/msvcr80.spec
index 9f92ba36a9f..91781c9c9e4 100644
--- a/dlls/msvcr80/msvcr80.spec
+++ b/dlls/msvcr80/msvcr80.spec
@@ -766,7 +766,7 @@
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
 @ extern _mbctype MSVCRT_mbctype
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat_s(ptr long str)
diff --git a/dlls/msvcr90/msvcr90.spec b/dlls/msvcr90/msvcr90.spec
index 8d647d6e907..2af41f8aea2 100644
--- a/dlls/msvcr90/msvcr90.spec
+++ b/dlls/msvcr90/msvcr90.spec
@@ -744,7 +744,7 @@
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
 @ extern _mbctype MSVCRT_mbctype
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat_s(ptr long str)
diff --git a/dlls/msvcrt/mbcs.c b/dlls/msvcrt/mbcs.c
index cc006765662..9b52170d4e9 100644
--- a/dlls/msvcrt/mbcs.c
+++ b/dlls/msvcrt/mbcs.c
@@ -2984,21 +2984,34 @@ unsigned char* CDECL _mbspbrk(const unsigned char *str, const unsigned char *acc
  */
 
 /*********************************************************************
- *		mblen(MSVCRT.@)
+ *		_mblen_l(MSVCRT.@)
  * REMARKS
  *  Unlike most of the multibyte string functions this function uses
  *  the locale codepage, not the codepage set by _setmbcp
  */
+int CDECL _mblen_l(const char* str, size_t size, _locale_t locale)
+{
+    pthreadlocinfo locinfo;
+
+    if (!str || !*str || !size)
+        return 0;
+
+    if (locale)
+        locinfo = locale->locinfo;
+    else
+        locinfo = get_locinfo();
+
+    if (locinfo->mb_cur_max == 1)
+        return 1; /* ASCII CP */
+    return !_isleadbyte_l((unsigned char)*str, locale) ? 1 : (size > 1 ? 2 : -1);
+}
+
+/*********************************************************************
+ *		mblen(MSVCRT.@)
+ */
 int CDECL mblen(const char* str, size_t size)
 {
-  if (str && *str && size)
-  {
-    if(get_locinfo()->mb_cur_max == 1)
-      return 1; /* ASCII CP */
-
-    return !isleadbyte((unsigned char)*str) ? 1 : (size>1 ? 2 : -1);
-  }
-  return 0;
+    return _mblen_l(str, size, NULL);
 }
 
 /*********************************************************************
diff --git a/dlls/msvcrt/msvcrt.spec b/dlls/msvcrt/msvcrt.spec
index 11b65d869dc..f54a437692f 100644
--- a/dlls/msvcrt/msvcrt.spec
+++ b/dlls/msvcrt/msvcrt.spec
@@ -711,7 +711,7 @@
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
 @ extern _mbctype MSVCRT_mbctype
-# stub _mblen_l(str long ptr)
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat(str str)
diff --git a/dlls/ucrtbase/ucrtbase.spec b/dlls/ucrtbase/ucrtbase.spec
index 4bb7f4148c7..7cf9b01aa4f 100644
--- a/dlls/ucrtbase/ucrtbase.spec
+++ b/dlls/ucrtbase/ucrtbase.spec
@@ -609,7 +609,7 @@
 @ cdecl _mbctombb_l(long ptr)
 @ cdecl _mbctoupper(long)
 @ cdecl _mbctoupper_l(long ptr)
-@ stub _mblen_l
+@ cdecl _mblen_l(str long ptr)
 @ cdecl _mbsbtype(str long)
 @ cdecl _mbsbtype_l(str long ptr)
 @ cdecl _mbscat_s(ptr long str)
@@ -1179,7 +1179,7 @@
 @ cdecl _o__mbctombb_l(long ptr) _mbctombb_l
 @ cdecl _o__mbctoupper(long) _mbctoupper
 @ cdecl _o__mbctoupper_l(long ptr) _mbctoupper_l
-@ stub _o__mblen_l
+@ cdecl _o__mblen_l(str long ptr) _mblen_l
 @ cdecl _o__mbsbtype(str long) _mbsbtype
 @ cdecl _o__mbsbtype_l(str long ptr) _mbsbtype_l
 @ cdecl _o__mbscat_s(ptr long str) _mbscat_s