"Peony" joined the Mediterranean Fleet after commissioning. In December 1915, the Royal Navy prepared to evacuate troops from the Dardanelles, with "Peony" serving as headquarters ship for General Julian Byng during the evacuation from Suvla Bay on the night of 18/19 December that year. In June 1916, "Peony" took part in a demonstration in Milo Bay to force the Greek Army to demobilise after Greece had allowed German forces to capture Fort Roupel and occupy much of eastern Macedonia without a fight.