Saints collapsed to a 4-1 defeat after falling behind for a second time and will now have to pick themselves up ahead of their Scottish Communities League Cup semi-final against Celtic next Sunday.
Richard Brittain fired the visitors ahead with an excellent 25-yard free-kick after 22 minutes and the Highland side passed up numerous opportunities to extend their advantage during the first half.
St Mirren levelled five minutes into the second half before Sam Morrow took advantage of a defensive howler by David Barron to restore County's lead.
Ivan Sproule added a third four minutes later and then capped off an excellent display when he tapped home a fourth.
"We looked the more likely after getting the equaliser," said Lennon.
"What frustrated me was we let the individual errors effect us and stopped doing what we were doing 10 seconds before the ball hit the net.
"It was a group of players I didn't really recognise, it rips me up inside to say Ross County wanted it more than we did.
"With the standards we set at the club and history we have, that is one of the worst performances I have seen."