He was a councillor for Whitehorse Manor in the then-County Borough of Croydon from 1949, having stood unsuccessfully in Thornton Heath ward in 1947.

As Labour Member of Parliament from 1957 for East Ham North, later Newham North East, he was a minister of state in Harold Wilson's first government at Education and Science (1964–66), then as Minister of Public Buildings and Works (1966–67), and finally was put in charge of the still-new Ministry of Overseas Development (1967–69).

At the next shadow cabinet election, Prentice topped the poll and he was again re-elected in 1973, this time finishing in third place.

[2] When Labour regained power, he was Secretary of State for Education and Science between 1974 and 1975, subsequently becoming Minister for Overseas Development with a seat in the cabinet until 1976.

[3][4] He appealed unsuccessfully from the rostrum of the Labour Party Conference for the National Executive Committee to overturn their endorsement of his deselection.