The Hoops looked to be cruising towards a comfortable win in Perth after second-half goals from Kris Commons, Teemu Pukki and Virgil van Dijk.
However, the Parkhead club switched off in the last 10 minutes and
allowed Saints players Scott Brown and Michael O'Halloran to score late goals.
The 3-3 draw means that the Scottish champions - who will receive the SPFL Premiership trophy on Sunday - can no longer break the 100-point barrier this season.
Lennon, who made six changes to his starting line-up, admitted he had a few harsh words for his charges at half-time in his side's penultimate league outing. But he defended their record since clinching their third consecutive title last month and insisted they would perform better against Dundee United this weekend.
He said: "We threw away two goals at the end which was disappointing. But we made the changes because we wanted to give players a game and there was a disjointed look about us in the first half. We didn't play well at all.
"We decided to make the changes at half-time and we should have seen the game out. But I think over the piece a draw was a fair result.
"I made my feelings known at half-time. It was miles off the level that we have been at. And I don't just mean the players who have come in. I mean the players who have been playing.
"I was unhappy with the defending for the final two goals. I thought we were lazy. But I can't be too critical of the way we have been playing.
"Since we have won the league we have won five and drawn two. We have really kept our momentum going really well. We have rotated the squad and had a look at some players."
Lennon gave rare starts to fringe players Amido Balde, Derk Boerritger, Teemu Pukki and keeper Lucasz Zaluska and was pleased with how his Finnish striker performed.
He said: "We have learned something. Teemu Pukki was excellent. He was our best player over the 90 minutes and thoroughly deserved his goal.
"He has been looking pretty good in training in the last few weeks. His first touch tonight was excellent as was his workrate. He was the only one who gave us any sort of positive impetus going forward.
"The other bonus was Mikael Lustig having 90 minutes for the first time in four months. He obviously tired towards the end which was understandable. But he will get a real benefit from that.
"He got stronger as the game went on. The majority of the team did. There was a fluency about us in the second half which was obviously missing from the first half."