Mahon was born into a Roman Catholic family of Irish descent in Bootle which was immersed in Liverpool Labour politics.
His great-nephew, Peter Dowd, is the present Labour MP for Bootle, and served as Shadow Chief Secretary to the Treasury until 2020.
Almost a decade later he was selected for the "bellwether" marginal seat of Preston South, a constituency with a significant Catholic population.
[2] On his election, Mahon introduced a Private Members Bill, to remove the necessity of applying for Probate in winding-up the estate of a poor person.
Peter Mahon was not selected, and instead stood in the by-election as an Independent "Labour and Anti-Abortion" candidate, securing a respectable 10.3% of the vote.