McLachlan returned from the injury on the opening day of the 1926–27 season, during which his side suffered a 4–3 defeat against Burnley. He enjoyed a prominent season for Cardiff, scoring 5 times in 38 league appearances. He also played in 5 of the club's 6 matches en route to reaching the 1927 FA Cup Final, scoring in the club's 2–0 victory over Darlington in the fourth round. Ahead of the final, the "Daily Herald" noted that McLachlan and Ernie Curtis had formed one of the "best wings in the First Division". Cardiff went on to win the final on 23 April 1927, defeating Arsenal 1–0 with a goal from Hughie Ferguson to become the only team from outside England to have won the competition in its history. McLachlan also helped the side complete a cup double by winning the Welsh Cup final two weeks later. He played in all five of the club's matches, scoring in the sixth and seventh rounds against Ebbw Vale and Barry Town United. In the final, Cardiff defeated Rhyl 2–0.