Tatchell was born in Melbourne, Australia. His father was a lathe operator and his mother worked in a biscuit factory. His parents divorced when he was four and his mother remarried soon afterwards. Since the family finances were strained by medical bills, he had to leave school at 16 in 1968.

In 1978, Tatchell joined the Labour Party and moved to a council flat in Bermondsey, south-east London. From October 1979, he became a leading member in a group of left-wingers planning to depose the right-wing caucus of Southwark councillors that controlled the Bermondsey Constituency Labour Party (CLP). At the CLP's AGM in February 1980, the left group won control and Tatchell was elected Secretary.  When the sitting Labour MP, Bob Mellish, announced his retirement in 1981, Tatchell was selected as his successor. The selection was a surprise, as Arthur Latham, a former MP and former Chairman of the Tribune Group, was the favourite. Later, the Militant group was cited as the reason for Tatchell's selection, but he has said that it had only a handful of members at that time in the constituency; he had never been a member and Militant did not support his selection. Tatchell ascribed his selection to the support of the "older, 'born and bred' working class; the younger professional and intellectual members swung behind Latham".  Due to Tatchell's support for direct action in the London Labour Briefing newsletter, Tatchell was denounced by party leader Michael Foot for allegedly supporting extra-parliamentary action against the Thatcher government; according to Tony Benn, Foot lied about Tatchell's alleged extremism in order to allow the Social Democratic Party to rejoin the Labour Party. Neil Kinnock stated that the whole affair was a matter of political judgement, asking "the question is: are we talking of extra-parliamentary or anti-parliamentary behaviour?" The fact that Tatchell was a gay man was also considered by some as a factor as to why Tatchell should not be supported. Labour subsequently allowed him to stand in the Bermondsey by-election, held in February 1983.

Did he hold any other positions?
Tatchell was denounced by party leader Michael Foot for allegedly supporting extra-parliamentary action against the Thatcher government;