diff --git a/mock/mock.go b/mock/mock.go
index a13c37f3b..a1f941bf2 100644
--- a/mock/mock.go
+++ b/mock/mock.go
@@ -390,8 +390,7 @@ func (m *Mock) findExpectedCall(method string, arguments ...interface{}) (int, *
 
 	for i, call := range m.ExpectedCalls {
 		if call.Method == method {
-			_, diffCount := call.Arguments.Diff(arguments)
-			if diffCount == 0 {
+			if call.Arguments.matchCount(arguments) == 0 {
 				expectedCall = call
 				if call.Repeatability > -1 {
 					return i, call
@@ -405,7 +404,6 @@ func (m *Mock) findExpectedCall(method string, arguments ...interface{}) (int, *
 
 type matchCandidate struct {
 	call      *Call
-	mismatch  string
 	diffCount int
 }
 
@@ -430,16 +428,14 @@ func (c matchCandidate) isBetterMatchThan(other matchCandidate) bool {
 	return false
 }
 
-func (m *Mock) findClosestCall(method string, arguments ...interface{}) (*Call, string) {
+func (m *Mock) findClosestCall(method string, arguments ...interface{}) *Call {
 	var bestMatch matchCandidate
 
 	for _, call := range m.expectedCalls() {
 		if call.Method == method {
-
-			errInfo, tempDiffCount := call.Arguments.Diff(arguments)
+			tempDiffCount := call.Arguments.matchCount(arguments)
 			tempCandidate := matchCandidate{
 				call:      call,
-				mismatch:  errInfo,
 				diffCount: tempDiffCount,
 			}
 			if tempCandidate.isBetterMatchThan(bestMatch) {
@@ -448,7 +444,7 @@ func (m *Mock) findClosestCall(method string, arguments ...interface{}) (*Call,
 		}
 	}
 
-	return bestMatch.call, bestMatch.mismatch
+	return bestMatch.call
 }
 
 func callString(method string, arguments Arguments, includeArgumentValues bool) string {
@@ -512,10 +508,13 @@ func (m *Mock) MethodCalled(methodName string, arguments ...interface{}) Argumen
 		//   a) this is a totally unexpected call to this method,
 		//   b) the arguments are not what was expected, or
 		//   c) the developer has forgotten to add an accompanying On...Return pair.
-		closestCall, mismatch := m.findClosestCall(methodName, arguments...)
+		closestCall := m.findClosestCall(methodName, arguments...)
 		m.mutex.Unlock()
 
 		if closestCall != nil {
+			// Format the diff outside the mutex to avoid deadlocks when
+			// arguments implement Stringer and call back into MethodCalled.
+			mismatch, _ := closestCall.Arguments.Diff(arguments)
 			m.fail("\n\nmock: Unexpected Method Call\n-----------------------------\n\n%s\n\nThe closest call I have is: \n\n%s\n\n%s\nDiff: %s\nat: %s\n",
 				callString(methodName, arguments, true),
 				callString(methodName, closestCall.Arguments, true),
@@ -953,6 +952,66 @@ func (args Arguments) Is(objects ...interface{}) bool {
 	return true
 }
 
+// matchCount returns the number of argument differences without formatting
+// output strings. This is safe to call while holding a mutex because it
+// does not invoke user-defined methods like String() or GoString() that
+// could call back into MethodCalled and cause a deadlock.
+func (args Arguments) matchCount(objects []interface{}) int {
+	maxArgCount := len(args)
+	if len(objects) > maxArgCount {
+		maxArgCount = len(objects)
+	}
+
+	var differences int
+	for i := 0; i < maxArgCount; i++ {
+		if len(objects) <= i || len(args) <= i {
+			differences++
+			continue
+		}
+		actual := objects[i]
+		expected := args[i]
+
+		if matcher, ok := expected.(argumentMatcher); ok {
+			func() {
+				defer func() {
+					if recover() != nil {
+						differences++
+					}
+				}()
+				if !matcher.Matches(actual) {
+					differences++
+				}
+			}()
+		} else {
+			switch expected := expected.(type) {
+			case anythingOfTypeArgument:
+				if reflect.TypeOf(actual).Name() != string(expected) && reflect.TypeOf(actual).String() != string(expected) {
+					differences++
+				}
+			case *IsTypeArgument:
+				if reflect.TypeOf(actual) != expected.t {
+					differences++
+				}
+			case *FunctionalOptionsArgument:
+				var name string
+				if len(expected.values) > 0 {
+					name = "[]" + reflect.TypeOf(expected.values[0]).String()
+				}
+				if name != reflect.TypeOf(actual).String() && len(expected.values) != 0 {
+					differences++
+				} else if ef, af := assertOpts(expected.values, actual); ef != "" || af != "" {
+					differences++
+				}
+			default:
+				if !assert.ObjectsAreEqual(expected, Anything) && !assert.ObjectsAreEqual(actual, Anything) && !assert.ObjectsAreEqual(actual, expected) {
+					differences++
+				}
+			}
+		}
+	}
+	return differences
+}
+
 // Diff gets a string describing the differences between the arguments
 // and the specified objects.
 //
diff --git a/mock/mock_test.go b/mock/mock_test.go
index 3dc9e0b1e..b1144e276 100644
--- a/mock/mock_test.go
+++ b/mock/mock_test.go
@@ -2462,6 +2462,35 @@ func TestIssue1785ArgumentWithMutatingStringer(t *testing.T) {
 	m.AssertExpectations(t)
 }
 
+// TestIssue1719StringerDeadlock verifies that MethodCalled does not deadlock
+// when an argument's String() method calls back into MethodCalled.
+// See https://github.com/stretchr/testify/issues/1719
+func TestIssue1719StringerDeadlock(t *testing.T) {
+	done := make(chan struct{})
+
+	go func() {
+		defer close(done)
+
+		m := &Mock{}
+		m.On("String").Return("")
+		m.On("DoAThing", Anything).Return()
+
+		// When DoAThing is called with the mock itself as an argument,
+		// Diff used to format the argument with %v, triggering String(),
+		// which calls MethodCalled("String") — deadlock because the mutex
+		// is already held by the outer MethodCalled("DoAThing").
+		m.MethodCalled("DoAThing", m)
+		m.MethodCalled("String")
+	}()
+
+	select {
+	case <-done:
+		// Success — no deadlock
+	case <-time.After(5 * time.Second):
+		t.Fatal("MethodCalled deadlocked when argument's String() calls MethodCalled")
+	}
+}
+
 func TestIssue1227AssertExpectationsForObjectsWithMock(t *testing.T) {
 	mockT := &MockTestingT{}
 	AssertExpectationsForObjects(mockT, Mock{})