Joined Celtic from Duntocher Hibernian in October 1926. He started as a left half but converted to left back. He made his Celtic debut again Falkirk on 27 August 1927.
Making his Scotland debut against England on 1 April 1933 at the age of 28, he went on to earn six caps. He never scored for Scotland, but missed a penalty kick in a 1-2 defeat by Ireland in his second game on 16 September 1933. His last Scotland cap came against Wales on 21 November 1934. He also represented the Scottish League.
His Celtic career often blighted with incidents came to an end in the summer of 1936 when he was released after playing 325 games scoring 8 goals. One such incident happened in a game against Rangers in 1935, when McGonagle infuriated by a late challenge on Joe Kennaway from Rangers’ big forward Jimmy Smith, which had laid out the Celtic keeper. Smith had a reputation for his overtly-physical approach to the game and on this occasion McGonagle thought he had gone too far. With Kennaway receiving treatment and Smith sat in the penalty area, the enraged McGonagle picked up the match ball and marched over to where the Rangers forward was sat. Standing directly over Smith, he slowly raised his arms and with considerable force bounced the ball off Smith’s head. McGonagle was sent off for his actions and for some Celtic directors it was an indiscretion too far for Peter and from that moment his days at Parkhead looked numbered.