Richard Brittain fired the visitors ahead with an excellent 25-yard curling free-kick and the Highland side passed up numerous opportunities to extend their advantage during the first half.
Saints were rewarded for a bright start to the second half when Steven Thompson rocketed a shot past Mark Brown from just inside the area but it was to be the highlight of the afternoon for Lennon's side.
Sam Morrow took advantage on a horrific defensive error by David Barron to restore County's lead and Ivan Sproule marked his debut with a goal four minutes later.
Sproule capped off an excellent display when he tapped home a fourth after Craig Samson could only parry a Rocco Quinn strike and a miserable day for St Mirren was completed when Lewis Guy's penalty was saved by Mark Brown in injury time.
Lennon felt that the performance was one of the worst that he had witnessed since taking charge in June 2010.
"I'm proud of my players prior to the game, they've got a focus to go and show the nation live on Sky what we're all about at this football club and we were very much a shadow of ourselves," Lennon said. "I do believe in this group of players and we certainly had a bad day at the office.
"The key thing to our success over recent months has been our endeavour, our commitment, our belief and more importantly the work-rate, which has been absolutely incredible up until this game.
"There was a lack of passion, a lack of desire, a lack of fight and it really hurts me."