St Mirren scored three first-half goals courtesy of a Ben Williams own goal and efforts from Adam Campbell and Steven Thompson; Hibs rallied but a second-half James Collins' brace was to be in vain.
Lennon, whose side moved to within five points of seventh-placed Hibs, said: "I was delighted with what we brought to the game in the first half - the hunger and desire.
"We played some fantastic stuff and scored three fantastic goals. We came in at half-time and you could sense one or two had not been in this position before.
"You try and pre-warn them not to change anything, but it happens. From the first half to the second half it was top drawer with different types of performances.
"I thought to a man we were at it in the first half and in the second half we pulled those sleeves up. I'm delighted our young pups handled that experience well."
The result marked a first home defeat for Terry Butcher since he replaced Pat Fenlon in November - and he was far from happy with what he witnessed.
"That was one of the most wretched 45 minutes I have seen from one of my teams," he said.
"That was absolutely abysmal and there were a few choice words at half-time.
"It tells me the squad depth isn't there. We only had six subs because some were injured and I felt others didn't deserve to be on the bench.
"There's a few players in the first half whose performances said to me 'I'm not good enough gaffer, get someone else in'.
"I can't do that in January, but I'll b****y well do that in the summer. That was not acceptable. That was awful.
"They were going to have Monday off, but I've got them in now and I'm going to work them hard. It's baffling as to where that performance came from."