diff --git a/libraries/mbed/common/Timer.cpp b/libraries/mbed/common/Timer.cpp
index d4ed133125..e00eaaf543 100644
--- a/libraries/mbed/common/Timer.cpp
+++ b/libraries/mbed/common/Timer.cpp
@@ -23,8 +23,10 @@ Timer::Timer() : _running(), _start(), _time() {
 }
 
 void Timer::start() {
-    _start = us_ticker_read();
-    _running = 1;
+    if (!_running) {
+        _start = us_ticker_read();
+        _running = 1;
+    }
 }
 
 void Timer::stop() {