Richard Morris, G4S regional chief executive of UK and Ireland, has resigned at time when the Serious Fraud Office is still investigating the troubled security giant.
G4S confirmed in a statement that Morris had left the company and been replaced by Eddie Ashton, who has risen up from his chief operating officer role.
A spokesman for the firm would not give details of why Morris left the role, which he had only assumed in October 2012.
Morris had been at G4S for more than a decade.
A number of scandals has surrounded G4S in recent years. It is the subject of an on-going probe by the Serious Fraud Office (SFO) on its alleged overcharging of the Ministry of Justice on its contracts to tag offenders.
Justice Minister Chris Grayling claimed that G4S - and its rival Serco - had charged the MoJ for tagging criminals who were dead, still in prison, or where no tag had actually been put on the offender.
Morris had been managing director of electronic monitoring at G4S Care & Justice Services.
During the London 2012 Olympic Games, G4S suffered embarrassment over its mishandling of its contract to handle security at the event. It failed to recruit enough staff in time, leaving the military and police to step in and pick up the slack. The blunder damaged G4S's reputation and cost it £88m.
It ultimately led to the departure of G4S group chief executive Nick Buckles, who admitted to MPs that the Olympics fiasco had been "humiliating".
Buckles was replaced by Ashley Almanza, who had been the G4S chief financial officer for just three weeks before his promotion.
At a shareholder meeting, Almanza said one of his "principal objectives is to try and put the Olympics and some of the setbacks behind us".
He also hinted at cost-slashing and job cuts for G4S.
"Let me reassure you that while the short term outlook is challenging the underlying business and the medium and long term outlook remain very strong indeed," said Almanza.
"We are putting in place a number of business improvement plans to strengthen margins in 2014 and 2015."
In November 2011 G4S dropped a £5.2bn bid to take over outsourcing giant ISS, amid shareholder concern over the size of the deal.