diff --git a/git-mergetool--lib.sh b/git-mergetool--lib.sh
index 54cb708254d380..f013a0350665e8 100644
--- a/git-mergetool--lib.sh
+++ b/git-mergetool--lib.sh
@@ -104,13 +104,49 @@ run_merge_tool () {
 
 	if merge_mode
 	then
-		merge_cmd "$1"
+		run_merge_cmd "$1"
 	else
-		diff_cmd "$1"
+		run_diff_cmd "$1"
 	fi
 	return $status
 }
 
+# Run a either a configured or built-in diff tool
+run_diff_cmd () {
+	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+	if test -n "$merge_tool_cmd"
+	then
+		( eval $merge_tool_cmd )
+		status=$?
+		return $status
+	else
+		diff_cmd "$1"
+	fi
+}
+
+# Run a either a configured or built-in merge tool
+run_merge_cmd () {
+	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
+	if test -n "$merge_tool_cmd"
+	then
+		trust_exit_code="$(git config --bool \
+			mergetool."$1".trustExitCode || echo false)"
+		if test "$trust_exit_code" = "false"
+		then
+			touch "$BACKUP"
+			( eval $merge_tool_cmd )
+			status=$?
+			check_unchanged
+		else
+			( eval $merge_tool_cmd )
+			status=$?
+		fi
+		return $status
+	else
+		merge_cmd "$1"
+	fi
+}
+
 list_merge_tool_candidates () {
 	if merge_mode
 	then
diff --git a/mergetools/defaults b/mergetools/defaults
index 1d8f2a3dd3d037..21e63ecc3e0192 100644
--- a/mergetools/defaults
+++ b/mergetools/defaults
@@ -8,36 +8,12 @@ can_diff () {
 }
 
 diff_cmd () {
-	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
-	if test -z "$merge_tool_cmd"
-	then
-		status=1
-		break
-	fi
-	( eval $merge_tool_cmd )
-	status=$?
+	status=1
 	return $status
 }
 
 merge_cmd () {
-	merge_tool_cmd="$(get_merge_tool_cmd "$1")"
-	if test -z "$merge_tool_cmd"
-	then
-		status=1
-		break
-	fi
-	trust_exit_code="$(git config --bool \
-		mergetool."$1".trustExitCode || echo false)"
-	if test "$trust_exit_code" = "false"
-	then
-		touch "$BACKUP"
-		( eval $merge_tool_cmd )
-		status=$?
-		check_unchanged
-	else
-		( eval $merge_tool_cmd )
-		status=$?
-	fi
+	status=1
 	return $status
 }
 
diff --git a/t/t7610-mergetool.sh b/t/t7610-mergetool.sh
index 6fa0c70506b4a4..bc38737b2a5ebd 100755
--- a/t/t7610-mergetool.sh
+++ b/t/t7610-mergetool.sh
@@ -509,4 +509,17 @@ test_expect_success 'file with no base' '
     git reset --hard master >/dev/null 2>&1
 '
 
+test_expect_success 'custom commands override built-ins' '
+    git checkout -b test14 branch1 &&
+    git config mergetool.defaults.cmd "cat \"\$REMOTE\" >\"\$MERGED\"" &&
+    git config mergetool.defaults.trustExitCode true &&
+    test_must_fail git merge master &&
+    git mergetool --no-prompt --tool defaults -- both &&
+    echo master both added >expected &&
+    test_cmp both expected &&
+    git config --unset mergetool.defaults.cmd &&
+    git config --unset mergetool.defaults.trustExitCode &&
+    git reset --hard master >/dev/null 2>&1
+'
+
 test_done
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 9c3e997b9d6be6..eb1d3f85b59b2e 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -76,6 +76,17 @@ test_expect_success PERL 'custom commands' '
 	test "$diff" = "branch"
 '
 
+# Ensures that a custom difftool.<tool>.cmd overrides built-ins
+test_expect_success PERL 'custom commands override built-ins' '
+	restore_test_defaults &&
+	git config difftool.defaults.cmd "cat \$REMOTE" &&
+
+	diff=$(git difftool --tool defaults --no-prompt branch) &&
+	test "$diff" = "master" &&
+
+	git config --unset difftool.defaults.cmd
+'
+
 # Ensures that git-difftool ignores bogus --tool values
 test_expect_success PERL 'difftool ignores bad --tool values' '
 	diff=$(git difftool --no-prompt --tool=bad-tool branch)