He was raised as a strict Catholic by the Order of Saint Anthony. His wife and children were killed in Montreal in a terrorist attack by the Martian Axis. He hated the Ice Warriors after that, since the Axis claimed to be sanctioned by them.
Five years later, he was the leader of an expedition to the Martian North Pole to investigate unusual energy readings. He was extremely distrustful of a group of Ice Warrior pilgrims they met, who were also heading to the North Pole. He came to realise, though, that the Ice Warriors had nothing to do with the Martian Axis, and was able to let go of his animus to the "Greenies". He even attended the wedding of Sstaal and Esstar.
However, his death was part of the Web of Time. The Seventh Doctor demonstrated this to Roz Forrester and Chris Cwej by showing them a list of the inhabitants of Mars who had died during the ten-year occupation of the solar system, which included McGuire. (PROSE: GodEngine)