Ben Marshall swept the visitors ahead seven minutes into the second half and only a fine block from Mark Little denied him a second before Jamie Vardy was denied by a Bobby Olejnik wonder save.
It was that failure to finish the job which annoyed Pearson as Davide Petrucci sparked a terrific turnaround with a 25-yard half-volley with 16 minutes still on the clock.
And the comeback was completed with two minutes to go when McCann classily fired past Kasper Schmeichel from the edge of the box.
Pearson admitted: "There's no doubt it goes down as a missed opportunity. When you get in front in a game like this, you have to close it out and we failed.
"We certainly created enough good chances to do that but I felt we didn't work hard enough to keep a clean sheet at the other end.
"That is not something you will hear me say about my players too often but I felt Peterborough had more drive than us, especially after they had got themselves level.
"You have to give them some credit for the way they finished the match."