In February 1973, Jock Stein paid £35,000 to sign Lynch for Celtic and, although Lynch was initially ineffective as a winger at Parkhead due to a chronic pelvic injury, Stein successfully converted him into a left back.
After Jock Stein's departure from Celtic Park in 1978, Lynch continued at left-back (and as captain in the absence of Danny McGrain) during Billy McNeill's first season in charge of the club.