The victory was only their second in the 17 games since beating Hearts in the League Cup final in March and it moved them 12 points above the bottom side in the Scottish Premiership.
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 Hearts striker Callum Paterson fired good chances over the bar just before each goal.
Lennon put their poor start to the season down to confidence and feels they can further build on their last two games.
"Getting back to the little things being faithful to the little things, and working hard, when you do that as a group, you get rewards for the big things," he added.
"I was under a bit of pressure last year with six straight defeats round about November. We managed to overcome that and turn it around.
"We managed to lift a bit of silverware and get one or two of the club's best results in their history. We went on to win three games in a row.
"I certainly believe with that type of performance, that character, that discipline, that hunger, that energy, there is no reason why we can't do it again."
Lennon, who is in his fourth season as St Mirren manager, added: "They have set the standard in the last two games and we set the standard in long spells last season, and we have set standards since my time here.
"This is a lean spell we were going through. There is no easy way to success. There is no elevator to success.
"There is only the stairway and we have taken another couple of steps."
- Sports & Recreation
- St Mirren