diff --git a/.git-blame-ignore-revs b/.git-blame-ignore-revs
index 5d3c35edb4d4..9156c6563ec7 100644
--- a/.git-blame-ignore-revs
+++ b/.git-blame-ignore-revs
@@ -193,6 +193,9 @@ cffc27daf06c77c0d76bc35d24b929cb9d68c3c9
 # nixos/kanidm: inherit lib, nixfmt
 8f18393d380079904d072007fb19dc64baef0a3a
 
+# fetchhg: format after refactoring with lib.extendMkDerivation and make overridable (#423539)
+34a5b1eb23129f8fb62c677e3760903f6d43228f
+
 # fetchurl: nixfmt-rfc-style
 ce21e97a1f20dee15da85c084f9d1148d84f853b
 
diff --git a/doc/build-helpers/fetchers.chapter.md b/doc/build-helpers/fetchers.chapter.md
index 96b85d7b0d4f..8ff1fdf11a19 100644
--- a/doc/build-helpers/fetchers.chapter.md
+++ b/doc/build-helpers/fetchers.chapter.md
@@ -840,7 +840,7 @@ Used with CVS. Expects `cvsRoot`, `tag`, and `hash`.
 
 ## `fetchhg` {#fetchhg}
 
-Used with Mercurial. Expects `url`, `rev`, and `hash`.
+Used with Mercurial. Expects `url`, `rev`, `hash`, overridable with [`<pkg>.overrideAttrs`](#sec-pkg-overrideAttrs).
 
 A number of fetcher functions wrap part of `fetchurl` and `fetchzip`. They are mainly convenience functions intended for commonly used destinations of source code in Nixpkgs. These wrapper fetchers are listed below.
 
diff --git a/pkgs/build-support/fetchhg/default.nix b/pkgs/build-support/fetchhg/default.nix
index dd6a8cfa87be..1e5e76ec4ca0 100644
--- a/pkgs/build-support/fetchhg/default.nix
+++ b/pkgs/build-support/fetchhg/default.nix
@@ -3,39 +3,48 @@
   stdenvNoCC,
   mercurial,
 }:
-{
-  name ? null,
-  url,
-  rev ? null,
-  sha256 ? null,
-  hash ? null,
-  fetchSubrepos ? false,
-  preferLocalBuild ? true,
-}:
 
-if hash != null && sha256 != null then
-  throw "Only one of sha256 or hash can be set"
-else
-  # TODO: statically check if mercurial as the https support if the url starts with https.
-  stdenvNoCC.mkDerivation {
-    name = "hg-archive" + (lib.optionalString (name != null) "-${name}");
-    builder = ./builder.sh;
-    nativeBuildInputs = [ mercurial ];
+lib.extendMkDerivation {
+  constructDrv = stdenvNoCC.mkDerivation;
 
-    impureEnvVars = lib.fetchers.proxyImpureEnvVars;
+  extendDrvArgs =
+    finalAttrs:
+    {
+      name ? null,
+      url,
+      rev ? null,
+      sha256 ? null,
+      hash ? null,
+      fetchSubrepos ? false,
+      preferLocalBuild ? true,
+    }:
+    # TODO: statically check if mercurial as the https support if the url starts with https.
+    {
+      name = "hg-archive" + (lib.optionalString (name != null) "-${name}");
+      builder = ./builder.sh;
+      nativeBuildInputs = [ mercurial ];
 
-    subrepoClause = lib.optionalString fetchSubrepos "S";
+      impureEnvVars = lib.fetchers.proxyImpureEnvVars;
 
-    outputHashAlgo = if hash != null then null else "sha256";
-    outputHashMode = "recursive";
-    outputHash =
-      if hash != null then
-        hash
-      else if sha256 != null then
-        sha256
-      else
-        lib.fakeSha256;
+      subrepoClause = lib.optionalString fetchSubrepos "S";
 
-    inherit url rev;
-    inherit preferLocalBuild;
-  }
+      outputHashAlgo = if finalAttrs.hash != null && finalAttrs.hash != "" then null else "sha256";
+      outputHashMode = "recursive";
+      outputHash =
+        lib.throwIf (finalAttrs.hash != null && sha256 != null) "Only one of sha256 or hash can be set"
+          (
+            if finalAttrs.hash != null then
+              finalAttrs.hash
+            else if sha256 != null then
+              sha256
+            else
+              ""
+          );
+
+      inherit url rev hash;
+      inherit preferLocalBuild;
+    };
+
+  # No ellipsis
+  inheritFunctionArgs = false;
+}
diff --git a/pkgs/test/overriding.nix b/pkgs/test/overriding.nix
index 27f57ab1ab9d..29bdbf2fd8fd 100644
--- a/pkgs/test/overriding.nix
+++ b/pkgs/test/overriding.nix
@@ -5,7 +5,7 @@
 }:
 
 let
-  tests = tests-stdenv // test-extendMkDerivation // tests-go // tests-python;
+  tests = tests-stdenv // test-extendMkDerivation // tests-fetchhg // tests-go // tests-python;
 
   tests-stdenv =
     let
@@ -131,6 +131,51 @@ let
       };
     };
 
+  tests-fetchhg =
+    let
+      ruamel_0_18_14-hash = "sha256-HDkPPp1xI3uoGYlS9mwPp1ZjG2gKvx6vog0Blj6tBuI=";
+      ruamel_0_18_14-src = pkgs.fetchhg {
+        url = "http://hg.code.sf.net/p/ruamel-yaml/code";
+        rev = "0.18.14";
+        hash = ruamel_0_18_14-hash;
+      };
+      ruamel_0_17_21-hash = "sha256-6PV0NyPQfd+4RBqoj5vJaOHShx+TJVHD2IamRinU0VU=";
+      ruamel_0_17_21-src = pkgs.fetchhg {
+        url = "http://hg.code.sf.net/p/ruamel-yaml/code";
+        rev = "0.17.21";
+        hash = ruamel_0_17_21-hash;
+      };
+      ruamel_0_17_21-src-by-overriding = ruamel_0_18_14-src.overrideAttrs {
+        rev = "0.17.21";
+        hash = ruamel_0_17_21-hash;
+      };
+    in
+    {
+      hash-outputHash-equivalence = {
+        expr = ruamel_0_17_21-src.outputHash == ruamel_0_17_21-hash;
+        expected = true;
+      };
+
+      hash-overridability-outputHash = {
+        expr = ruamel_0_17_21-src-by-overriding.outputHash == ruamel_0_17_21-hash;
+        expected = true;
+      };
+
+      hash-overridability-drvPath = {
+        expr =
+          lib.isString ruamel_0_17_21-src-by-overriding.drvPath
+          && ruamel_0_17_21-src-by-overriding.drvPath == ruamel_0_17_21-src.drvPath;
+        expected = true;
+      };
+
+      hash-overridability-outPath = {
+        expr =
+          lib.isString ruamel_0_17_21-src-by-overriding.outPath
+          && ruamel_0_17_21-src-by-overriding.outPath == ruamel_0_17_21-src.outPath;
+        expected = true;
+      };
+    };
+
   tests-go =
     let
       pet_0_3_4 = pkgs.buildGoModule rec {