Shrivastava secured 35,265 votes (40.4%), ahead of Labour on 30,374 votes (34.8%), while Reform UK received 7,288 votes (8.4%).
The Liberal Democrats took 7.3%, the Conservatives 5.3% and an independent candidate 2.5%.
The result marks a significant political shift in a borough long considered a Labour stronghold.
Lewisham has been Labour-controlled for decades and has had a directly elected mayor since 2002.
Outgoing mayor Brenda Dacres had been serving after winning a 2024 by-election.
The result represents a major gain for the Green Party in south east London.
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Shrivastava is a councillor for New Cross Gate and a member of the Lewisham Green Party Group.
Lewisham has seen significant regeneration in areas including Deptford, Lewisham town centre and Catford, with housing and development key issues for voters.
The mayor holds executive powers over housing, planning and council spending.
