diff --git a/esphome/core/scheduler.h b/esphome/core/scheduler.h
index 07456dc4fe..8897027461 100644
--- a/esphome/core/scheduler.h
+++ b/esphome/core/scheduler.h
@@ -290,7 +290,7 @@ class Scheduler {
   // because scheduling only compares millis_64 against millis_64 — never against millis().
   // On other platforms (ESP8266), extends now to 64-bit using rollover tracking, so both
   // millis() and scheduling use the same clock.
-  uint64_t millis_64_from_(uint32_t now) {
+  uint64_t ESPHOME_ALWAYS_INLINE millis_64_from_(uint32_t now) {
 #ifdef USE_NATIVE_64BIT_TIME
     (void) now;
     return millis_64();
diff --git a/esphome/core/time_64.cpp b/esphome/core/time_64.cpp
index db5df25eb9..41917e825e 100644
--- a/esphome/core/time_64.cpp
+++ b/esphome/core/time_64.cpp
@@ -20,6 +20,12 @@ namespace esphome {
 static const char *const TAG = "time_64";
 #endif
 
+#ifdef ESPHOME_THREAD_SINGLE
+// Storage for Millis64Impl inline compute() — defined here so all TUs share one copy.
+uint32_t Millis64Impl::last_millis_{0};
+uint16_t Millis64Impl::millis_major_{0};
+#else
+
 uint64_t Millis64Impl::compute(uint32_t now) {
   // Half the 32-bit range - used to detect rollovers vs normal time progression
   static constexpr uint32_t HALF_MAX_UINT32 = std::numeric_limits<uint32_t>::max() / 2;
@@ -44,51 +50,25 @@ uint64_t Millis64Impl::compute(uint32_t now) {
    * to last_millis is provided by its release store and the corresponding acquire loads.
    */
   static std::atomic<uint16_t> millis_major{0};
-#elif !defined(ESPHOME_THREAD_SINGLE) /* ESPHOME_THREAD_MULTI_NO_ATOMICS */
+#else /* ESPHOME_THREAD_MULTI_NO_ATOMICS */
   static Mutex lock;
   static uint32_t last_millis{0};
   static uint16_t millis_major{0};
-#else                                 /* ESPHOME_THREAD_SINGLE */
-  static uint32_t last_millis{0};
-  static uint16_t millis_major{0};
 #endif
 
   // THREAD SAFETY NOTE:
-  // This function has three implementations, based on the precompiler flags
-  // - ESPHOME_THREAD_SINGLE - Runs on single-threaded platforms (ESP8266, etc.)
+  // This function has two out-of-line implementations, based on the precompiler flags:
   // - ESPHOME_THREAD_MULTI_NO_ATOMICS - Runs on multi-threaded platforms without atomics (LibreTiny BK72xx)
   // - ESPHOME_THREAD_MULTI_ATOMICS - Runs on multi-threaded platforms with atomics (LibreTiny RTL87xx/LN882x, etc.)
   //
+  // The ESPHOME_THREAD_SINGLE path is inlined in time_64.h.
   // Make sure all changes are synchronized if you edit this function.
   //
   // IMPORTANT: Always pass fresh millis() values to this function. The implementation
   // handles out-of-order timestamps between threads, but minimizing time differences
   // helps maintain accuracy.
 
-#ifdef ESPHOME_THREAD_SINGLE
-  // Single-core platforms have no concurrency, so this is a simple implementation
-  // that just tracks 32-bit rollover (every 49.7 days) without any locking or atomics.
-
-  uint16_t major = millis_major;
-  uint32_t last = last_millis;
-
-  // Check for rollover
-  if (now < last && (last - now) > HALF_MAX_UINT32) {
-    millis_major++;
-    major++;
-    last_millis = now;
-#ifdef ESPHOME_DEBUG_SCHEDULER
-    ESP_LOGD(TAG, "Detected true 32-bit rollover at %" PRIu32 "ms (was %" PRIu32 ")", now, last);
-#endif /* ESPHOME_DEBUG_SCHEDULER */
-  } else if (now > last) {
-    // Only update if time moved forward
-    last_millis = now;
-  }
-
-  // Combine major (high 32 bits) and now (low 32 bits) into 64-bit time
-  return now + (static_cast<uint64_t>(major) << 32);
-
-#elif defined(ESPHOME_THREAD_MULTI_NO_ATOMICS)
+#if defined(ESPHOME_THREAD_MULTI_NO_ATOMICS)
   // Without atomics, this implementation uses locks more aggressively:
   // 1. Always locks when near the rollover boundary (within 10 seconds)
   // 2. Always locks when detecting a large backwards jump
@@ -202,6 +182,8 @@ uint64_t Millis64Impl::compute(uint32_t now) {
 #endif
 }
 
+#endif  // !ESPHOME_THREAD_SINGLE
+
 }  // namespace esphome
 
 #endif  // !USE_NATIVE_64BIT_TIME
diff --git a/esphome/core/time_64.h b/esphome/core/time_64.h
index 42d4b041e5..592e645d41 100644
--- a/esphome/core/time_64.h
+++ b/esphome/core/time_64.h
@@ -4,6 +4,9 @@
 #ifndef USE_NATIVE_64BIT_TIME
 
 #include <cstdint>
+#include <limits>
+
+#include "esphome/core/helpers.h"
 
 namespace esphome {
 
@@ -16,7 +19,36 @@ class Millis64Impl {
   friend uint64_t millis_64();
   friend class Scheduler;
 
+#ifdef ESPHOME_THREAD_SINGLE
+  // Storage defined in time_64.cpp — declared here so the inline body can access them.
+  static uint32_t last_millis_;
+  static uint16_t millis_major_;
+
+  static inline uint64_t ESPHOME_ALWAYS_INLINE compute(uint32_t now) {
+    // Half the 32-bit range - used to detect rollovers vs normal time progression
+    static constexpr uint32_t HALF_MAX_UINT32 = std::numeric_limits<uint32_t>::max() / 2;
+
+    // Single-core platforms have no concurrency, so this is a simple implementation
+    // that just tracks 32-bit rollover (every 49.7 days) without any locking or atomics.
+    uint16_t major = millis_major_;
+    uint32_t last = last_millis_;
+
+    // Check for rollover
+    if (now < last && (last - now) > HALF_MAX_UINT32) {
+      millis_major_++;
+      major++;
+      last_millis_ = now;
+    } else if (now > last) {
+      // Only update if time moved forward
+      last_millis_ = now;
+    }
+
+    // Combine major (high 32 bits) and now (low 32 bits) into 64-bit time
+    return now + (static_cast<uint64_t>(major) << 32);
+  }
+#else
   static uint64_t compute(uint32_t now);
+#endif
 };
 
 }  // namespace esphome