diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8670b99fb0a..ac4b73c18d9 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 * [FEATURE] Compactor: Add `-compactor.skip-blocks-with-out-of-order-chunks-enabled` configuration to mark blocks containing index with out-of-order chunks for no compact instead of halting the compaction
 * [FEATURE] Querier/Query-Frontend: Add `-querier.per-step-stats-enabled` and `-frontend.cache-queryable-samples-stats` configurations to enable query sample statistics
 * [FEATURE] Add shuffle sharding for the compactor #4433
+* [BUGFIX] Distributor: Fix race condition on `/series` introduced by #4683. #4716
 
 ## 1.12.0 in progress
 
diff --git a/pkg/distributor/distributor.go b/pkg/distributor/distributor.go
index 95e3650002f..25fede59ec2 100644
--- a/pkg/distributor/distributor.go
+++ b/pkg/distributor/distributor.go
@@ -980,12 +980,14 @@ func (d *Distributor) MetricsForLabelMatchers(ctx context.Context, from, through
 		return nil, err
 	}
 
+	mutex.Lock()
 	result := make([]metric.Metric, 0, len(metrics))
 	for _, m := range metrics {
 		result = append(result, metric.Metric{
 			Metric: m,
 		})
 	}
+	mutex.Unlock()
 	return result, nil
 }
 
diff --git a/pkg/distributor/distributor_test.go b/pkg/distributor/distributor_test.go
index b01855a8180..63cbefb930e 100644
--- a/pkg/distributor/distributor_test.go
+++ b/pkg/distributor/distributor_test.go
@@ -1841,6 +1841,36 @@ func TestSlowQueries(t *testing.T) {
 	}
 }
 
+func TestDistributor_MetricsForLabelMatchers_SingleSlowIngester(t *testing.T) {
+	// Create distributor
+	ds, ing, _, _ := prepare(t, prepConfig{
+		numIngesters:        3,
+		happyIngesters:      3,
+		numDistributors:     1,
+		shardByAllLabels:    true,
+		shuffleShardEnabled: true,
+		shuffleShardSize:    3,
+		replicationFactor:   3,
+	})
+
+	ing[2].queryDelay = 50 * time.Millisecond
+
+	ctx := user.InjectOrgID(context.Background(), "test")
+
+	now := model.Now()
+
+	for i := 0; i < 100; i++ {
+		req := mockWriteRequest([]labels.Labels{{{Name: labels.MetricName, Value: "test"}, {Name: "app", Value: "m"}, {Name: "uniq8", Value: strconv.Itoa(i)}}}, 1, now.Unix())
+		_, err := ds[0].Push(ctx, req)
+		require.NoError(t, err)
+	}
+
+	for i := 0; i < 50; i++ {
+		_, err := ds[0].MetricsForLabelMatchers(ctx, now, now, mustNewMatcher(labels.MatchEqual, model.MetricNameLabel, "test"))
+		require.NoError(t, err)
+	}
+}
+
 func TestDistributor_MetricsForLabelMatchers(t *testing.T) {
 	const numIngesters = 5
 
@@ -2192,10 +2222,10 @@ type prepConfig struct {
 	errFail                      error
 }
 
-func prepare(tb testing.TB, cfg prepConfig) ([]*Distributor, []mockIngester, []*prometheus.Registry, *ring.Ring) {
-	ingesters := []mockIngester{}
+func prepare(tb testing.TB, cfg prepConfig) ([]*Distributor, []*mockIngester, []*prometheus.Registry, *ring.Ring) {
+	ingesters := []*mockIngester{}
 	for i := 0; i < cfg.happyIngesters; i++ {
-		ingesters = append(ingesters, mockIngester{
+		ingesters = append(ingesters, &mockIngester{
 			happy:      *atomic.NewBool(true),
 			queryDelay: cfg.queryDelay,
 		})
@@ -2206,7 +2236,7 @@ func prepare(tb testing.TB, cfg prepConfig) ([]*Distributor, []mockIngester, []*
 			miError = cfg.errFail
 		}
 
-		ingesters = append(ingesters, mockIngester{
+		ingesters = append(ingesters, &mockIngester{
 			queryDelay: cfg.queryDelay,
 			failResp:   *atomic.NewError(miError),
 		})
@@ -2225,7 +2255,7 @@ func prepare(tb testing.TB, cfg prepConfig) ([]*Distributor, []mockIngester, []*
 			RegisteredTimestamp: time.Now().Add(-2 * time.Hour).Unix(),
 			Tokens:              []uint32{uint32((math.MaxUint32 / cfg.numIngesters) * i)},
 		}
-		ingestersByAddr[addr] = &ingesters[i]
+		ingestersByAddr[addr] = ingesters[i]
 	}
 
 	kvStore, closer := consul.NewInMemoryClient(ring.GetCodec(), log.NewNopLogger(), nil)
@@ -2637,6 +2667,7 @@ func (i *mockIngester) QueryStream(ctx context.Context, req *client.QueryRequest
 }
 
 func (i *mockIngester) MetricsForLabelMatchers(ctx context.Context, req *client.MetricsForLabelMatchersRequest, opts ...grpc.CallOption) (*client.MetricsForLabelMatchersResponse, error) {
+	time.Sleep(i.queryDelay)
 	i.Lock()
 	defer i.Unlock()
 
@@ -3098,7 +3129,7 @@ func TestDistributor_Push_RelabelDropWillExportMetricOfDroppedSamples(t *testing
 	require.NoError(t, testutil.GatherAndCompare(regs[0], strings.NewReader(expectedMetrics), metrics...))
 }
 
-func countMockIngestersCalls(ingesters []mockIngester, name string) int {
+func countMockIngestersCalls(ingesters []*mockIngester, name string) int {
 	count := 0
 	for i := 0; i < len(ingesters); i++ {
 		if ingesters[i].countCalls(name) > 0 {