Mayne's name was recommended to Captain David Stirling by his friend Lt. Eoin McGonigal, a fellow officer of No. 11 (Scottish) Commando, and an early volunteer for the Special Air Service (SAS) – then known simply as the Parachute Unit. It is widely believed that Mayne was under arrest for hitting his commanding officer, Lieutenant Colonel Geoffrey Charles Tasker Keyes when Stirling met him. This story is untrue. A hand-written entry in Keyes' personal diary proves conclusively that he was not at the officer's mess of No. 11 (Scottish) Commando at Salamis on Cyprus on the evening of 21 June 1941, the date on which Mayne was accused of beating up a fellow officer, Major Charles Napier. Keyes had stayed the night elsewhere, and arrived at Salamis the following day, 22 June 1941, when the trouble was already over. Keyes states in his diary that he conducted an investigation and found Mayne responsible.