McLachlan, a powerful half-back, hailed from Dumfries in Scotland. His brother Andy, four years older than Bert and an inside-forward, was also a professional footballer. Andy played for St Catherine's Boys Club, Partick Thistle and Dundee, before signing for English First Division giants Aston Villa in August 1912.

Bert began his playing days with home town team St Cuthbert's Wanderers and then joined up with his brother at Villa Park in 1913. He was limited to three League appearances for Villa before returning to Scotland with Aberdeen a year later. The First World War ate into the best years of McLachlan's playing days, though he played wartime football for Everton and spent the 1918/19 season with Leeds City while based in a nearby Army camp.

His Peacocks debut came on the opening day, 7 September 1918, at Elland Road against Notts County, when the clubs fought out a 1-1 draw. McLachlan made 24 appearances that season for City before returning to Aberdeen to captain the Dons during 1919/20, when he was a mainstay of the team at left-half. His displays earned him a place in the Scottish League side that lost 4-0 to the Football League at Celtic Park on 20 March 1920.

McLachlan left Pittodrie to join Heart of Midlothian in 1927 after 361 appearances for Aberdeen.

He died in Edinburgh in 1956.