The striker, who left Posh to join the Tigers last year, netted the winner in the second half to help Hull to their second consecutive league victory.
Peterborough lacked a cutting edge in the final third and, as a result, suffered their second successive loss and their first home defeat of the season.
Grant McCann, who has three league goals this campaign, had the first chance of the match as early as the second minute but he just fired over from
England Under-21 striker Martyn Waghorn, making his full Hull debut, almost broke the deadlock after seven minutes but his effort was palmed onto the crossbar by Paul Jones and, after saving Robert Koren's rebound, the Posh goalkeeper gathered the ball from a tame Paul McKenna effort.
Robbie Brady fired wide from distance for the visitors, before Peterborough came back into the game and, after 25 minutes, captain McCann tried his luck one again, but his 25-yard strike was well saved by Adriano Basso.
Striker George Boyd, a constant thorn in the Tigers defence, struck from distance but Basso, despite fumbling the ball, managed to cling on at the second attempt.
Waghorn almost took advantage of a defensive error but the on-loan Leicester forward was thwarted by a crucial defensive block from Gabriel Zakuani, before Boyd, at the other end, had a shot blocked from James Chester and defender Mark Little failed to hit the target from 25 yards.
The only goal of the game came just two minutes after the interval. Tom Cairney's inswinging corner fell kindly to the quiet Mclean, who made no mistake, volleying across Jones inside the far upright to give Nigel Pearson's side the lead.
On-loan Manchester United winger Brady could have doubled Hull's advantage but he once again failed to get his shot on target.
Taylor fired over from the edge of the box and Tomlin failed to direct his header on target as Posh searched for an equaliser, while substitute Tommy Rowe, coming on at the start of the second half for Lee Frecklington, had a shot deflected over the bar.
Ryan Bennett fired into the side netting.with the goal at his mercy before Hull went up the other end and almost doubled their advantage but Jones had to be alert to tip Jack Hobbs' header round the post.
New signing Emile Sinclair almost snatched an equaliser for the hosts but Basso superbly denied the former Macclesfield frontman and Craig Alcock flashed a shot across the face of the goal.
Despite four minutes added time, Darren Ferguson's side failed to find a leveller.