McGrain played in eight consecutive internationals for Scotland and was selected by Willie Ormond for the 22 man squad travelling to West Germany for the 1974 World Cup. McGrain played in all three World Cup group matches (Zaire, Brazil and Yugoslavia), but despite being undefeated Scotland returned home having failed to qualify for the next phase. This early-exit from the tournament proved fortunate for McGrain, who had been suffering from extreme thirst all through the tournament and had lost 2 stone (12 kg) in weight. On his return to Glasgow, McGrain was diagnosed as being diabetic. Had there been any further delay in identification and treatment of his condition, the consequences could have proved fatal. With his illness now being managed, McGrain was able to continue playing football.