Mclean is no stranger to being a match-winner at London Road only this time it was in the colours of the Tigers.
Mclean returned to haunt his former club by firing the only goal of a tight npower Championship tussle two minutes after the interval.
It was just the fourth goal Mclean has scored for Hull since making a £1.3million move from Posh back in January, but enough to condemn his old employers to a first home league defeat of the campaign.
He did his best to stick to a pre-match pledge of not celebrating despite being mobbed by his delighted team-mates and Hull Pearson was another man thrilled to see his striker find the net.
He said: "I'm really pleased for Aaron. Anyone who saw him train this week will have known how desperate he was to come back to Peterborough and play well.
"He hasn't scored the number of goals he would have liked since coming to the club, but he contributes so much to the team in his all-round performances.
"I'm also delighted for the other lads to come here and get the result we wanted despite a far from ideal preparation.
"We had eight lads away on international duty and I felt we looked tired in the first half, but we did enough to get the result."
Peterborough manager Darren Ferguson said: "I had a feeling Aaron might score when that ball dropped to him, but there is no way we deserved to lose a game that probably should have ended 0-0.
"I'm well aware of the fact we haven't scored since hitting seven the other week, but that's not a concern.
"We are still creating chances and have to continue to believe in what we are doing. We won't change the way we play because we have the right philosophy here."
Hull - often criticised for not scoring enough goals - would have hit the front far earlier but for the early brilliance of Paul Jones.
The Posh keeper pulled off a fine triple save in the seventh minute, turning a curler from Hull debutant Martyn Waghorn onto the bar before also denying follow-ups from Robert Koren and Paul McKenna.
The visitors had received a let-off even earlier when Posh captain Grant McCann landed a first-minute 25-yard drive onto the roof of the net.
McCann tested Hull keeper Adriano Basso with another effort from distance later in the first half, but it was not until late on that the hosts really began to look dangerous.
Half-time arrival Tommy Rowe should have done better than firing into the side-netting before Basso pulled off a stunning save to keep out a goalbound George Boyd snap-shot.
But Posh had to endure a third successive match without scoring since they dished out a 7-1 televised thumping to Ipswich last month.