After working as Political Secretary of the Royal Arsenal Co-operative Society (R.A.C.S.), serving as a Greenwich borough councillor and unsuccessfully contesting the seats of Bexley (1970, Edward Heath) and Bexleyheath (February 1974, Cyril Townsend), in October 1974 John Cartwright was elected as the Labour member for Woolwich East. He replaced Christopher Mayhew who had left Labour to join the Liberal Party. Following six years as a backbench Member of Parliament and latterly Parliamentary Private Secretary to Shirley Williams, Cartwright himself left the Labour Party in 1981 to become one of the founding members of the SDP.
John Cartwright served as the SDP's chief whip from 1983 onwards and as its President from 1987 until the failure of the continuing SDP in 1990. He also served as the SDP/Liberal Alliance's chief defence spokesman from 1983 to 1987. A close political ally of David Owen, he stayed loyal to Owen and Owen's continuing SDP after the Liberal Party and a majority of the SDP merged in January 1988 to become the Liberal Democrats.
Following the collapse of the continuing SDP in 1990, Cartwright stood for re-election as an Independent Social Democrat - albeit one endorsed by the Liberal Democrats - but lost by 2,200 votes. After thus leaving active politics he went on to serve as Deputy Chairman of the Police Complaints Authority before retiring to Kent.