Liberal Democrat Sally Hamwee will share the chairman's post with Trevor Phillips, who ran as Frank Dobson's running mate in the elections.
She will also chair the GLA's budget committee, which will have the power to challenge Ken Livingstone's decisions.
Baroness Hamwee, who lives in Mortlake, represented the Palewell ward for 20 years and was the first Liberal Democrat to chair the council's planning committee.
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