County should not have needed over 120 minutes to settle the game as they dominated for long spells, but were harshly punished in St Mirren's rare forays forward.
Two penalty saves from Lee Mair and John Potter left Ross County goalkeeper Michael McGovern taking the plaudits, but playmaker Michael Gardyne again proved a capable tormentor when it came to twisting and weaving his way around the St Mirren defence.
Lennon's side had the lead at the interval against the run of play.
County goalkeeper McGovern dived at the feet of Michael Higdon to turn the ball round for a corner but from Paul McGowan's cross, Darren McGregor headed the opener from the edge of the six yard box after 44 minutes.
On the half hour Gardyne forced Paul Gallacher to pull off the first save with a 20-yard drive.
Richard Brittain's corner found the head of Scott Boyd, with the St Mirren goalkeeper called into action once again to save on the line.
County continued to press after the break and, with the early change of Steven Craig for Garry Wood freshened up their attack.
Gardyne slid a well-measured pass for Craig to make a quick impact but he blasted well over.
Craig was involved again as County swept forward, the striker's deft header allowing Gary Miller to push on, but the County defender saw his cross blocked for a corner.
As the cross came in, Boyd was again foiled, as St Mirren's Marc McAusland cleared at the right post.
Parity was restored on 69 minutes when Gardyne blazed a trail through the St Mirren defence before firing an unstoppable drive at the left corner of the box.
County moved ahead on 78 minutes when Mair fouled Martin Scott to concede a free-kick 25 yards out.
County skipper Richard Brittain, formerly a St Mirren player, dispatched a precision finish into the top right corner.
The home side's joy was shortlived as less than 60 seconds later Michael Higdon sweetly volleyed St Mirren's equaliser from just inside the penalty area as a long ball caught County cold.
Extra-time also saw both sides caught out, as Jure Travner provided the cross for McGowan to score on 92 minutes, while at the other end a scuffed attempt by Mark Corcoran saw Steven Craig follow up to prod home from six yards.
County continued to search for a winner, with sub Iain Vigurs denied by Gallacher as his low drive was saved.
The home side were not to be denied and Vigurs converted his penalty in the shoot-out along with Brittain, Paul Lawson and man of the match Gardyne.