Born in South Shields, Slater went to sea in 1941, and joined the National Union of Seamen (NUS).
On one occasion his ship was torpedoed and sank, and in line with practice at the time, his pay was immediately stopped.
He was active in the seamen's strike of 1966, during which Harold Wilson implied he was one of several union activists under communist influence, a charge which Slater rejected.
[2] Tom Hadaway wrote the play "Seafarers" in 1993, based on a true story of Slater being arrested after his Geordie accent led to him being mistaken for a spy.
[2][6] In the last years of his life, Slater was involved in the campaign for a proper investigation of the sinking of the MV Derbyshire.