In 2000, Johnson was elected to the inaugural London Assembly as part of a three-strong Green Group, including Jenny Jones AM and Victor Anderson (who resigned in March 2003 and was replaced by Noel Lynch for the remainder of the term). The 2004 elections would see the Greens lose the seat held by Lynch, leaving Johnson and Jones as the two remaining members of the Green Group. Both won re-election in the 2008 election, when the Greens held on to their two seats. He served as Deputy Chair of the Assembly 2008-09 and was elected Chair of the Assembly in May 2009.
