 After the war, Probert joined the Labour Party, and from 1949 to 1954 was the secretary of the Aberdare Trades and Labour Council.From 1951, he was additionally secretary of the Glamorgan Federation of Trades Councils.Probert was first elected as an MP to the Commons at the 1954 Aberdare by-election, following the death of Labour MP David Emlyn Thomas.