Mountford, who served as Lord Sugar's aide for five series of the BBC One reality show, will be one of the four interviewers tasked with grilling the remaining candidates.
The corporate lawyer, who departed the show to study for a PhD in papyrology, will be joined in the task by Apprentice regular Claude Littner, Shortlist Media chief exec Mike Soutar and IT entrepreneur Matthew Riley.
Mountford returned for the Apprentice interview stage last year and famously lambasted Stuart Baggs for his informal approach to the challenge.
The interviews will take place in the show's final episode.
The Apprentice continues tomorrow night on BBC One with a wholesale goods task.
> 'Apprentice' to have four-candidate final
Watch a clip of Margaret Mountford from last year's Apprentice below: