 At the 1964 general election, he was elected as member of parliament for Preston South, defeating the sitting Conservative MP Alan Green by only 348 votes.He held the seat at the 1966 election with an increased majority, but at the 1970 general election, Green retook the seat with a majority of over 1,300.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.