diff --git a/pageserver/src/page_service.rs b/pageserver/src/page_service.rs
index 05e55e6092..65c3dac40e 100644
--- a/pageserver/src/page_service.rs
+++ b/pageserver/src/page_service.rs
@@ -2258,9 +2258,7 @@ impl PageServerHandler {
 
         let result: Option<SystemTime> = timeline
             .lease_standby_horizon(lease_id, lsn, ctx)
-            .inspect_err(|e| {
-                warn!("{e}");
-            })
+            // logging happens inside
             .ok();
 
         // Encode result as Option<millis since epoch>
@@ -3946,7 +3944,7 @@ impl proto::PageService for GrpcPageServiceHandler {
         // Attempt to acquire a lease. Return FailedPrecondition if the lease could not be granted.
         let expiration = match timeline.lease_standby_horizon(lease_id, lsn, &ctx) {
             Ok(expiration) => expiration,
-            Err(err) => return Err(tonic::Status::failed_precondition(format!("{err:#}"))),
+            Err(err) => return Err(tonic::Status::failed_precondition(format!("{err}"))),
         };
 
         Ok(tonic::Response::new(expiration.into()))
diff --git a/pageserver/src/tenant.rs b/pageserver/src/tenant.rs
index f576119db8..ee10b51eff 100644
--- a/pageserver/src/tenant.rs
+++ b/pageserver/src/tenant.rs
@@ -4830,6 +4830,7 @@ impl TenantShard {
                 // Cull any expired leases
                 let now = SystemTime::now();
                 target.leases.retain(|_, lease| !lease.is_expired(&now));
+                timeline.standby_horizons.cull_leases(now);
 
                 timeline
                     .metrics
diff --git a/pageserver/src/tenant/timeline.rs b/pageserver/src/tenant/timeline.rs
index de942c153b..ae062ca326 100644
--- a/pageserver/src/tenant/timeline.rs
+++ b/pageserver/src/tenant/timeline.rs
@@ -1861,13 +1861,27 @@ impl Timeline {
         Ok(lease)
     }
 
+    #[instrument(skip(_ctx), ret(Debug), err(Debug))]
     pub(crate) fn lease_standby_horizon(
         &self,
         lease_id: String,
         lsn: Lsn,
-        ctx: &RequestContext,
+        _ctx: &RequestContext,
     ) -> anyhow::Result<SystemTime> {
-        todo!()
+        if self
+            .feature_resolver
+            .evaluate_boolean("standby-horizon-leases-track-disable")
+            .is_ok()
+        {
+            // Use feature-flagging as a fail-safe in case something is wrong with the data structure (memory consumption, etc.)
+            return Ok(SystemTime::now()
+                .checked_add(Duration::from_secs(5 * 60))
+                .unwrap());
+        }
+        let length = todo!("duplicate init lease deadline logic?");
+        self.standby_horizons
+            .upsert_lease(lease_id, lsn, length)
+            .map(|lease| lease.valid_until)
     }
 
     /// Freeze the current open in-memory layer. It will be written to disk on next iteration.
diff --git a/pageserver/src/tenant/timeline/standby_horizon.rs b/pageserver/src/tenant/timeline/standby_horizon.rs
index fdda479643..dc6d67f353 100644
--- a/pageserver/src/tenant/timeline/standby_horizon.rs
+++ b/pageserver/src/tenant/timeline/standby_horizon.rs
@@ -10,6 +10,11 @@
 //! - legacy: as described in RFC36, replica->safekeeper->broker->pageserver
 //! - leases: TODO
 
+use std::{
+    collections::{HashMap, hash_map},
+    time::{Duration, SystemTime},
+};
+
 use metrics::IntGauge;
 use utils::lsn::Lsn;
 
@@ -18,7 +23,31 @@ pub struct Horizons {
 }
 struct Inner {
     legacy: Option<Lsn>,
-    pub legacy_metric: IntGauge,
+    legacy_metric: IntGauge,
+    leases_by_id: HashMap<String, Lease>,
+}
+
+struct Lease {
+    valid_until: SystemTime,
+    lsn: Lsn,
+}
+
+impl Lease {
+    pub fn try_update(&mut self, update: Lease) -> anyhow::Result<()> {
+        let Lease {
+            valid_until: expiration,
+            lsn,
+        } = update;
+        anyhow::ensure!(self.valid_until <= expiration);
+        anyhow::ensure!(self.lsn <= lsn);
+        *self = update;
+        Ok(())
+    }
+}
+
+#[derive(Debug)]
+pub struct LeaseInfo {
+    pub valid_until: SystemTime,
 }
 
 /// Returned by [`Self::min_and_clear_legacy`].
@@ -36,6 +65,7 @@ impl Horizons {
             inner: std::sync::Mutex::new(Inner {
                 legacy: None,
                 legacy_metric,
+                leases_by_id: Default::default(),
             }),
         }
     }
@@ -62,11 +92,37 @@ impl Horizons {
             inner.legacy.take()
         };
 
-        // TODO: support leases
-        let leases = [];
-
-        let all = legacy.into_iter().chain(leases.into_iter()).min();
+        let all = legacy
+            .into_iter()
+            .chain(inner.leases_by_id.values().map(|lease| lease.lsn))
+            .min();
 
         Mins { legacy, all }
     }
+
+    pub fn upsert_lease(
+        &self,
+        id: String,
+        lsn: Lsn,
+        length: Duration,
+    ) -> anyhow::Result<LeaseInfo> {
+        let mut inner = self.inner.lock().unwrap();
+        let valid_until = SystemTime::now() + length;
+        let update = Lease { valid_until, lsn };
+        let updated = match inner.leases_by_id.entry(id) {
+            hash_map::Entry::Occupied(mut entry) => {
+                entry.get_mut().try_update(update)?;
+                entry.into_mut()
+            }
+            hash_map::Entry::Vacant(entry) => entry.insert(update),
+        };
+        Ok(LeaseInfo {
+            valid_until: updated.valid_until,
+        })
+    }
+
+    pub fn cull_leases(&self, now: SystemTime) {
+        let mut inner = self.inner.lock().unwrap();
+        inner.leases_by_id.retain(|_, l| l.valid_until <= now);
+    }
 }