More than 400 people have been fined for not wearing face coverings on Transport for London services in the last week, Sadiq Khan said on Friday.
Tube and bus users in the capital risk a £200 penalty if caught not wearing a mask as the battle against the spread of the Omicron Covid variant is stepped up.
The Mayor of London said British Transport Police were helping TfL and the Metropolitan Police issue fines.
Mr Khan kept compulsory mask wearing rules on all London transport service after the Government dropped the legal requirement in the summer.
But no fines were issued for not complying.
“As far as TfL is concerned between July 10 and 10 days ago, zero fines were issued because we cannot issue fines,” Mr Khan told Sky News.
“In the last week we’ve issued more than 400 fines but the good news is [British Transport] Police are helping us enforce as well.”
He added that since the Government brought in rules that masks had to be worn on public transport compliance had gone up from 50 to 90 per cent in London.
From Friday face masks became mandatory in more settings including cinemas, theatres and churches as the Government rolls out “Plan B” restrictions.
The tougher measures - which from Monday will also include vaccine passports and working from home - are designed to slow the spread of the new Omicron variant.
The UK Health Security Agency reported on Wednesday night that the “majority” of the 705 suspected cases of the more active variant were in London and the South East, including more than 200 in the capital.
