Spending on City Hall employees has risen by 82% while Sadiq Khan has been mayor, according to official figures.
The Conservatives said the mayor was "shelling out millions on waste".
A spokesperson for the mayor said the higher costs were due to "additional powers across a wide range of areas" which had been granted to Mr Khan.
City Hall budgets show staff costs have increased each year while Mr Khan has been in charge.
Employee numbers have also risen during the four-year period, from 897 full-time equivalent posts in 2016 to about 1,300, as proposed in the 2020-21 budget.
The leader of the Conservatives on Greater London Authority, Susan Hall, said the "surging staff numbers are completely disproportionate to the minuscule increase in the extra responsibilities which have been devolved to the mayor".
She said: "There can be no question that amount of taxpayers' money being spent on City Hall bureaucrats is excessive and desperately needs to be reduced."
A spokesperson for the mayor said the additional powers included areas such as housing, skills and healthcare while the extra staff "in many cases... have been funded by money the mayor has secured from the government".
"The mayor makes no apologies whatsoever for putting the resources in place to ensure he can continue delivering real results in all of these priority areas," the spokesperson said.