Chris White, of Harold Hill in east London, is part of a “Stop Ulez Expansion” campaign.
He argued that Mr Khan’s decision to expand Ulez was unlawful – but a judge has ruled against him.
Lawyers representing Mr Khan said Mr White did not have an arguable case and Mrs Justice Cockerill on Thursday ruled in their favour.
Mrs Justice Cockerill considered arguments at a High Court hearing in London.
Mr White argued that Mr Khan’s “stated objective” – to “clean up London’s toxic air and tackle the climate emergency” – was unlawful because it had not been mandated by Parliament.
He was not at Thursday’s hearing, but lawyers representing him indicated that they were considering an appeal.
A group of Conservative-led councils unsuccessfully challenged Mr Khan’s expansion plans earlier this year.
The outer London boroughs of Bexley, Bromley, Harrow, and Hillingdon – plus Surrey County Council – had challenged proposals to extend Ulez beyond the North and South Circular roads.
Another High Court judge dismissed their challenge in July.