Pegasus supported the British intervention in the Russian Civil War between May and September 1919 and was based at Archangel. The ship returned to Rosyth and was briefly decommissioned. She recommissioned on 2 December 1919 and was transferred to the Mediterranean Fleet in March 1920. Pegasus ran aground on 9 March off Kerch, but was pulled off without suffering any significant damage.
HMS Pegasus was laid down in 1914 by the John Brown & Company of Clydebank, Scotland as Stockholm for the Great Eastern Railway Company's Harwich–Hook of Holland service, but her construction was suspended by the beginning of the First World War. The ship was purchased by the Royal Navy on 27 February 1917 and was launched on 9 June 1917. She was commissioned on 14 August 1917 and completed on 28 August 1917.