Saints secured their first win of the season and only their second in the 17 games since beating Hearts in the League Cup final in March as a 2-0 success at Tynecastle moved them 12 points above the bottom side.
John McGinn headed a 42nd-minute opener following a positive first-half display from the visitors and Paul McGowan capitalised on an uncharacteristic mistake by Hearts goalkeeper Jamie MacDonald in the 58th minute.
Hearts had gone into the game looking to further cut the gap created by their 15-point administration penalty, but St Mirren, after weathering a lively first 10 minutes from their hosts, built impressively on their performance in picking up their second point of the season against Aberdeen on Monday night.
Their midfielders swarmed forward in an entertaining first half and they gave an assured defensive display after going two up, although Callum Paterson missed good chances just before each goal.
Lennon said: "The character has been in question, it has been thrown at us in recent weeks, myself and the character of my players, and I am glad at times like this that it's character that is required because I certainly know I have it in abundance in that dressing room.
"Where we have been, they say that lean spells in football are character-building exercises and I believe it reveals the character of the individual.
"I certainly know I'm a fighter and those guys in there are fighters.
"We were looking to take that wee bit of confidence from Monday night's game and I thought we carried that on and we finished that by picking up all three points.
"I'm delighted for them and the supporters.
"We were certainly going to be up against it from the outset in terms of the atmosphere, the media, the 15,000 fans that were in here, from the Hearts players.
"Everyone expected us to come here and crumble and we stood up and were counted.
"Everybody would have seen it as a story if Hearts had closed the gap on us and it writes a story for you guys about a comeback. Well there was another story written today, it was our story, our comeback, and we certainly showed a lot more of our identity."
Lennon has faced questions about his own future head on in recent weeks, but he insisted there were few discussions about his job with the directors.
"There have been a lot of questions thrown about my time as manager," he said.
"I discussed one meeting with my chairman over this, and that was three weeks ago.
"Other than that, it has been business as usual.
"I have a board of directors that are very passionate supporters. They feel the same anger and hurt as we do.
"But in the three years I have been here, the board know the character of Danny Lennon. They also know the character, hunger and the belief that they have in the group of players we have here."
Hearts manager Gary Locke admitted his young team had not reached the early-season standards that had raised fans' hopes that they could cut the gap at the bottom, which now stands at 10 points following Kilmarnock's defeat by Dundee United.
"I thought we started reasonably well, we were on the front foot, passing it well and then the last 25 minutes of the half we didn't play well enough," Locke said.
"It's as simple as that, I am not going to sit here and make excuses.
"But it's probably the first time this season that I could say that. The boys have been brilliant up till now."
Locke now has the task of lifting his players for their trip to Motherwell in a fortnight.
"I said before the game there is still a long way to go after today," Locke said.
"Obviously we would have liked to have won the game and put the pressure on St Mirren.
"There were defining moments in the game, Callum has a wee chance at 1-0 and they go right up the park and score.
"The boys are very down, but, with having such a young team, we said at the start of the season it was going to be a big ask and after today it's not going to be any different."