He served with the Second Australian Imperial Force in the South Pacific during World War II, and trained at Yanco Experimental Farm after repatriation.
He was chairman of the Windsor District Milk Zone Dairymen's Council, and a member of the New South Wales executive of the Primary Producers' Union.
Blacktown had been a safe Labor seat, but an electoral redistribution had made it marginal, and the sitting member, John Freeman had chosen to retire.
A further redistribution before the 1962 election erased Dennis' majority and made Blacktown notionally a marginal Labor seat.
Dennis subsequently resigned from the Liberal Party and contested The Hills as an independent, losing to Ruddock, while the defeated candidate from 1959, Southee, went on to win Blacktown on a large swing.