 After demobilisation, he became full-time Assistant Secretary of the Civil Service Clerical Association, a trade union.In 1947, he joined the Labour Party Research Department, and was for four years the secretary of the party's policy committee, which involved him in drafting various articles of party policy during this period.At the 1951 general election, Fienburgh was elected as the Member of Parliament (MP) for the Labour seat of Islington North, in North London, although at the time of his death, he was actually living in Hemel Hempstead.