Thomson was born Patrick McAleer to an unmarried Northern Irish mother in Salford, England. She placed him for adoption and returned to Ulster, and he was adopted as a baby by Andrew and Marita Thomson. He grew up near Preston, Lancashire and was very happy with his family.
He was never secretive about his adoption, but he did not want to trace his birth family. However one of the British gutter press newspapers took it upon themselves to do this in 2003, and published the results without his consent, which greatly upset him and his family.
He is best known as a comic actor, in the British television series The Fast Show, Cold Feet, Dog Eat Dog and Born Romantic.