McGrain made his full international debut for Scotland on 12 May 1973, in a British Home Championship tie against Wales at Wrexham, winning 2–0. He won a further two caps over the following week in Home Championship ties against Northern Ireland and England. The game against England saw McGrain fielded in an unfamiliar role at left-back to accommodate another highly rated right-back, Sandy Jardine of Rangers. McGrain would go on to play at left-back for many of his Scotland appearances in the 1970s in order to accommodate the Rangers player, still able to play to the abilities he displayed in his normal role.