Described by historian Alan Stephens as "acerbic but capable", McLachlan led No. 3 Squadron through the Battle of Sidi Barrani in December 1940, followed by the Battle of Bardia and the capture of Tobruk in January 1941. Prior to converting to Hawker Hurricanes that month, the unit was credited with destroying twelve Italian aircraft for the loss of five Gladiators and two pilots; McLachlan shot down a Fiat CR.42 on 10 December 1940, the same action in which fellow squadron member and future ace Gordon Steege claimed his first "kill". Air Officer Commanding-in-Chief RAF Middle East, Air Marshal Sir Arthur Longmore, praised McLachlan and his squadron for their "high morale and adaptability to desert conditions".