For Posh, the day started with a setback as a consortium including former England star Paul Gascoigne and headed by Imraan Ladak confirmed it will not be investing, despite an informal meeting with owner and former manager Barry Fry during the week.
Ladak, whose group is looking to put money into a suitable club, said in a statement: "We were very impressed by everything we saw at Peterborough but following discussions both parties have agreed not to pursue this avenue any further."
Mark Wright's approach was surprisingly defensive for a home game with five at the back which allowed Mansfield to dominate first half exchanges, although only Adam Rundle came close to breaking the deadlock with a rising shot that was well saved by Posh goalkeeper Mark Tyler.
At the other end, youngster Jason White, drafted in to replace the experienced goalkeeper Kevin Pressman, a late injury victim, was hardly troubled despite impressive teenager Jamie Day producing some dangerous crosses.
Posh were far more positive after the break and needed only five minutes to go in front with winger Farrell steering a header wide of White after St. Ledger was released by Adam Newton to fire over a fine cross was that was helped on by James Quinn.
Roles were reversed on the hour when Farrell was the provider with a strong centre that the highly rated St. Ledger, figuring in an unaccustomed wing back role, put into the back of the net with a towering header to claim his first league goal.
Richie Barker might have pulled one back for Mansfield in added time, but Tyler had all the angles covered.