Needing little short of a miracle to keep alive their own fleeting play-off hopes alive, Posh started brightly with Richard Logan wasting an early opening before hitting the Wycombe bar with a far better effort.
Easter, replacing rested leading scorer Tommy Mooney, put Wycombe ahead with a fine solo effort after 14 minutes, turning superbly at the edge of the area before lifting a right-foot shot over stranded goalkeeper Mark Tyler and into the top corner of the net.
After being given the usual London Road reception accorded to former Cambridge United players, Easter further endeared himself to the Posh faithful with an exuberant goal celebration.
Posh pulled themselves together and tried to play constructive football on an atrocious surface with James Quinn glancing a pin-point Ryan Semple cross inches wide. Then a solo break by one-time Wycombe favourite David Farrell ended with a fierce shot that Steve Williams blocked with his legs before Roger Johnson tidied up.
Williams excelled again after the break, keeping out a cracking effort from Northern Ireland international Quinn before Posh owner Barry Fry, back on the touchline as acting manager, put even more emphasis on attack, sending on Jamie Day and Lloyd Opara to join a four-man front line.
Joe Burnell fired wide as Wycombe gained a brief defensive respite, but the relentless pressure soon resumed. With Williams handling faultlessly, however, and Johnson solid at the heart of defence, Posh never managed to grab the goal their efforts merited and Williamson had the last word, forcing in an 85th minute header following a corner.