Having been chosen to accompany the main squad to the 1970 European Cup Final along with Dalglish, McGrain made his first-team debut for Celtic as a substitute in a Scottish League Cup tie against Dundee United at Tannadice on 26 August 1970. He then went on and made his league debut three days later in the opening fixture against Morton at Celtic Park. McGrain was used sparingly by manager Jock Stein in his first couple of seasons in the first team, but the young full-back played well in these games.