The game got off to a lively start with Aaron McLean's second minute header well saved by Barnet keeper Rob Beckwith.
But the Bees had a chance to take the lead on seven minutes when Cliff Akurang should have done better with his header after good work from Jason Puncheon and Albert Adomah.
As conditions deteriorated midway through the first half the visitors began to take hold of the game with George Boyd and skipper Micah Hyde bossing the midfield.
Chris Whelpdale volleyed narrowly wide for Posh before the game burst into life in the 32nd minute.
Akurang was booked after bringing down Whelpdale and Dean Keates' resultant free-kick eluded everyone before going in via the crossbar.
Things went from bad to worse for the home side two minutes later when Puncheon was sent off for kicking the ball at a linesman.
Peterborough dominated the rest of the half with the classy Boyd looking increasingly dangerous.
However, Barnet captain Ismail Yakubu was resolute at the heart of the home defence as they reached half-time without suffering further damage.
The second period started with the visitors pinning their hosts back in their own half, but Craig Mackail-Smith pulled their best chance wide.
Barnet would not lie down though and a flurry of bookings seemed to inspire the home side as Adomah hit the bar with a smart back header.
At the other end, Jamie Day rattled the crossbar with a free-kick after Kenny Gillet had brought down Mackail-Smith.
The industrious Adomah had another chance to equalise late on, but Day sealed the win with a stoppage-time free-kick to give Darren Ferguson's side a sixth straight win.