The investigation will look at the mayor's recruitment procedures
The London Assembly will investigate the mayor's appointment of staff and consultants following the resignation of two senior aides, it has said.
It will examine recruitment procedures and advice given to Mayor Boris Johnson about his powers of appointment.
Last week, Deputy Mayor for Young People Ray Lewis resigned amid claims of financial irregularity in the past.
Earlier Deputy Mayor Tim Parker admitted advisers should have their backgrounds checked more thoroughly.
Mr Lewis's resignation followed last month's resignation of the mayor's director of political strategy James McGrath.
The investigation will be carried out by the London Assembly's business management and administration committee.
Assembly Member John Biggs said: "To lose one adviser might be considered unfortunate but to lose two in two weeks looks like carelessness."
Mr Biggs said there were "important issues about the appointment of publicly-funded staff that need to be addressed".
Mr Lewis resigned after facing allegations relating to his time as a vicar in east London in the late 1990s and head of a youth academy scheme in 2003.
He was placed under Church of England disciplinary measures in 1999.
Mr Lewis resigned amid claims of financial irregularity
After Mr Lewis's resignation Mr Johnson said he had been wrongly led to believe that Mr Lewis was a magistrate.
On Wednesday, Mr Parker told the Assembly that background checks were carried out on candidates for senior posts but improvements needed to be made.
"What is difficult to contend with, and I think this is something which we will review, is incidences where people don't actually tell 100-and-whatever per cent of the truth," Mr Parker said.
But Assembly Member Mike Tuffrey said Mr Parker "failed to address the issues about appointments exposed by the recent resignations".