Born in Moresby, near Whitehaven, Stephenson left school at the age of fourteen and followed his father in working at the Walkmill Colliery.
Inspired by Tom Cape and various socialist speakers, Stephenson joined the Independent Labour Party (ILP) and became active in the Cumberland Miners' Association (CMA).
He was consistently re-elected to Ennerdale Rural District Council, and he was selected as the ILP candidate for Whitehaven at the 1935 general election.
However, he took only 3.3% of the votes cast and, although he remained loyal to the ILP, he began to focus his work in the labour movement.
He was elected to the national executive of the Miners' Federation of Great Britain in 1939, where he was the leading proponent of strikes during World War II.