Invesco Perpetual's Mark Barnett has taken over Neil Woodford's £8.4bn Income and £13.1bn High Income funds with immediate effect, ahead of his original end-April start date.
Woodford will remain with the firm until 29 April, as per the announcement made by Invesco last October.
"Our transition plans have progressed well since October 2013, and Mark is ready to assume responsibility for the funds," said Invesco CEO Mark Armour.
Armour added Woodford will continue running "a number of segregated mandates" for Invesco prior to his departure.
The manager will move to Oakley Capital in May to launch his Woodford Investment Management business, at which point he will set up his own UK equity income fund.
Oakley is set to launch the new fund in May via a three-week offer period. Invesco has previously said it expects most "noise and activity" in terms of redemptions from its own funds to occur at the time of Woodford's departure on 29 April.
Oakley, meanwhile, has already recruited a number of the manager's former colleagues to help establish the venture, including former Invesco Perpetual sales head Craig Newman.
Lack of innovation for solutions
Some 2,000 consumers affected
Achievements, charity work and other happy snippets
Appetite has suffered since Brexit vote
'Failure to pay attention can result in enforcement'