diff --git a/tsdb/head.go b/tsdb/head.go
index 4840f0cae5..fb5c4def9e 100644
--- a/tsdb/head.go
+++ b/tsdb/head.go
@@ -420,7 +420,8 @@ type headMetrics struct {
 	mmapChunkCorruptionTotal  prometheus.Counter
 	snapshotReplayErrorTotal  prometheus.Counter // Will be either 0 or 1.
 	oooHistogram              prometheus.Histogram
-	mmapChunksTotal           prometheus.Counter
+	mmapChunksTotal            prometheus.Counter
+	headChunksMaxPendingMmap   prometheus.Gauge
 	walReplayUnknownRefsTotal *prometheus.CounterVec
 	wblReplayUnknownRefsTotal *prometheus.CounterVec
 }
@@ -560,6 +561,10 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
 			Name: "prometheus_tsdb_mmap_chunks_total",
 			Help: "Total number of chunks that were memory-mapped.",
 		}),
+		headChunksMaxPendingMmap: prometheus.NewGauge(prometheus.GaugeOpts{
+			Name: "prometheus_tsdb_head_chunks_max_pending_mmap",
+			Help: "Maximum number of head chunks pending m-mapping observed in any individual series during the last m-map pass.",
+		}),
 		walReplayUnknownRefsTotal: prometheus.NewCounterVec(prometheus.CounterOpts{
 			Name: "prometheus_tsdb_wal_replay_unknown_refs_total",
 			Help: "Total number of unknown series references encountered during WAL replay.",
@@ -598,6 +603,7 @@ func newHeadMetrics(h *Head, r prometheus.Registerer) *headMetrics {
 			m.checkpointCreationTotal,
 			m.oooHistogram,
 			m.mmapChunksTotal,
+			m.headChunksMaxPendingMmap,
 			m.mmapChunkCorruptionTotal,
 			m.snapshotReplayErrorTotal,
 			// Metrics bound to functions and not needed in tests
@@ -1945,12 +1951,17 @@ func (h *Head) getOrCreateWithOptionalID(id chunks.HeadSeriesRef, hash uint64, l
 // since holding the lock during an append could delay the next scrape or cause query timeouts.
 func (h *Head) mmapHeadChunks() {
 	var count int
+	var maxPendingMmapChunks uint32
 	for i := range h.series.size {
 		h.series.locks[i].RLock()
 		for _, series := range h.series.series[i] {
-			if series.headChunkCount.Load() < 2 { // < 2 means 0 or 1 head chunks, nothing to mmap.
+			hcc := series.headChunkCount.Load()
+			if hcc < 2 { // < 2 means 0 or 1 head chunks, nothing to mmap.
 				continue
 			}
+			if hcc > maxPendingMmapChunks {
+				maxPendingMmapChunks = hcc
+			}
 
 			series.Lock()
 			count += series.mmapChunks(h.chunkDiskMapper)
@@ -1958,6 +1969,7 @@ func (h *Head) mmapHeadChunks() {
 		}
 		h.series.locks[i].RUnlock()
 	}
+	h.metrics.headChunksMaxPendingMmap.Set(float64(maxPendingMmapChunks))
 	h.metrics.mmapChunksTotal.Add(float64(count))
 }
 
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index d4279185fe..57735877a4 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -8231,12 +8231,14 @@ func TestHead_mmapHeadChunks(t *testing.T) {
 	require.Equal(t, uint32(1), getCount(lblsB), "headChunkCount should be 1 after mmap")
 	require.Equal(t, uint32(1), getCount(lblsC), "headChunkCount should be 1 after mmap")
 	require.Equal(t, 0, countReady(), "ready set should be empty after mmapHeadChunks")
+	require.Equal(t, float64(3), prom_testutil.ToFloat64(h.metrics.headChunksMaxPendingMmap), "max pending mmap should be 3 (both series had 3 head chunks)")
 
 	// A second call should be a no-op.
 	beforeMetric := prom_testutil.ToFloat64(h.metrics.mmapChunksTotal)
 	h.mmapHeadChunks()
 	afterMetric := prom_testutil.ToFloat64(h.metrics.mmapChunksTotal)
 	require.Equal(t, beforeMetric, afterMetric, "second call should mmap 0 chunks")
+	require.Equal(t, float64(0), prom_testutil.ToFloat64(h.metrics.headChunksMaxPendingMmap), "max pending mmap should be 0 when no series needs mmapping")
 
 	// Only newly ready series should be processed.
 	app = h.Appender(t.Context())
@@ -8257,6 +8259,7 @@ func TestHead_mmapHeadChunks(t *testing.T) {
 	afterMetric = prom_testutil.ToFloat64(h.metrics.mmapChunksTotal)
 	require.Greater(t, afterMetric, beforeMetric, "third call should mmap chunks from series B")
 	require.Equal(t, uint32(1), getCount(lblsB), "series B headChunkCount should be 1 after mmap")
+	require.Equal(t, float64(3), prom_testutil.ToFloat64(h.metrics.headChunksMaxPendingMmap), "max pending mmap should be 3 (only series B had 3 head chunks)")
 }
 
 func TestHead_mmapHeadChunks_oooDoesNotInflateCount(t *testing.T) {