Deegan was born in Bootle in 1910, one of eleven children, to an Irish father and a mother born in Bootle of Irish parents. He left school at 14 and sold papers during the general strike of 1926. After going away to sea, he returned to Liverpool and worked as a casual labourer on the docks.
Joining the Communist Party in 1931, Deegan was active in the National Unemployed Workers Movement. In early 1937, the British Union of Fascists organised a rally to be addressed by its leader, Oswald Mosley, at Liverpool boxing stadium in St. Paul’s Square. Frank Deegan was of the many who gathered to oppose the rally.
On 13th May 1937 Frank Deegan was handed a coach ticket to London by Jack Jones, the first step on a long journey for Spain and the International Brigade. After crossing the Pyrenees, Deegan joined the British Battalion on the Aragón front as it attacked Quinto. He was wounded at Purburell Hill, and again later during the Ebro offensive.
On his return from Spain, he was active on the docks and in the T&G, remaining a local activist until his death late in life.