Paul Gallagher marked his recall to the Leicester team with the opening goal after 56 minutes, but Tomlin brought the visitors level in the 71st minute.
It was the first time the Foxes had drawn at home in the Championship all season.
With captain Matt Mills suspended following his red card in the defeat at Hull last weekend, Republic of Ireland international Sean St Ledger was recalled to partner Sol Bamba at the heart of the Leicester defence.
And manager Nigel Pearson made a second change, with Gallagher replacing Neil Danns in midfield.
On-loan full-back Tom Kennedy was ineligible for Peterborough against his parent club so a defensive reshuffle saw Tommy Rowe fill in at left-back as Lee Frecklington was recalled in midfield in the only change for Posh.
Leicester dominated early on and might have taken the lead through headed efforts from Jermaine Beckford and Bamba, but both players missed the target from six yards out when it seemed easier to do better.
Joe Lewis, recalled in goal last week following seven months out with a knee injury, kept his place despite Peterborough conceding four goals in the defeat to Barnsley last weekend. He fumbled a long-range shot from Lloyd Dyer after 15 minutes, though, giving Leicester some hope.
Beckford was unlucky to see a shot from the edge of the area strike a post seven minutes before half-time.
Home stopper Kasper Schmeichel was called into action at the start of the second half to block a volley from Emile Sinclair.
Leicester then deservedly went ahead after 56 minutes.
Gallagher took a short corner to David Nugent, whose attempted cross ricocheted back into the path of the Scot and he drilled a right-foot shot past Lewis at the near post.
Peterborough improved after that and got themselves level midway through the second half.
Sinclair used his pace to get free down the right-hand side and his cross was headed home by Lee Tomlin diving into the six-yard box.
Beckford had a chance to restore Leicester's lead soon afterwards but the former Everton and Leeds striker shot over the crossbar when again it looked simpler to hit the target.
At the other end, Lewis did well to handle a fiercely-struck shot from Lee Peltier with 10 minutes remaining, but neither side was able to create a decent enough chance to win the game.