Tomlin had been expected to be out of action for a considerable length of time with an ankle problem, but defied the odds to be fit for the trip to Ashton Gate - and celebrated with a cracking 25-yard drive on 71 minutes to secure victory.
George Boyd had given Posh the lead just after half-time and, despite Marvin Elliott's late consolation, City remain rock bottom of the npower Championship and are still without a home win this season.
The Robins had the game's first chance, with Neil Kilkenny testing Peterborough keeper Paul Jones from range. The Posh stopper could only just hang onto the Australian's attempt, but smothered the danger.
And the home side enjoyed the better of the openings in the early exchanges, with Gabriel Zakuani doing well to block Brett Pitman's shot before Nicky Maynard crashed his follow-up into the side-netting.
Jones was called on again on 24 minutes, getting across well to keep out Pitman's header from Albert Adomah's cross, as the Robins began to dominate proceedings.
Elliott nodded over from Kilkenny's centre when well placed, Adomah was denied by Jones, and it took until the stroke of half-time for Peterborough to fashion a chance of note.
Tommy Rowe tested City keeper David James but the veteran was equal to the effort, saving with his body, and the former England number one also saved from Emile Sinclair to ensure the two teams went into the interval goalless.
After the break, the home side seemed to be taking control, as Damion Stewart went close from two quick corners before Elliott failed to direct his header goalwards.
But despite all the Robins' momentum, it was Posh who took the lead.
Lee Frecklington slipped in Boyd, who outmuscled Kalifa Cisse to find the time and space to slide the ball into the bottom corner.
The goal spurred the visitors on, and James had to be alert to stop Boyd doubling the lead moments later as City started rocking.
Having regained their composure, the home side again went close through Pitman and Lewin Nyatanga but lacked a clinical edge in the final third.
And Peterborough seemed to be having similar problems, as Frecklington and Zakuani both fired wild attempts over the top of James' crossbar, before Tomlin doubled their advantage out of nothing.
Tomlin, recalled to the starting line-up after a quick recovery from an ankle injury, produced a screaming finish to find the top corner from 25 yards and put Posh out of sight.
City reduced arrears with eight minutes remaining, as Elliott rose highest to nod substitute Jamal Campbell-Ryce's cross beyond Jones from close range, but it was too little too late.