Fittingly Leon McGee, their American guard who has suffered a large share of Riders' frustrations this season, snaked through Birmingham's defence to slip in the winner 13 seconds from time.
Riders' 26-year-old American coach, Bob Donewald, who signed a two-year contract on Friday, has endured a string of injuries and problems among his players, plus a volatile relationship with the electrifying McGee.
Donewald suspended McGee from the second leg of their League Trophy semi- final with Chester Jets in February, which the Riders lost by a single point. McGee, who had treatment on his long-term knee injury during Saturday's game, said: "It's been a disappointing and frustrating season.
"We've had our problems and everyone counted us out of achieving anything, but we stayed with it. Leopards might be league champions, but now we've wiped the slate clean and start fresh next weekend."
The axe was hanging over Riders' season on Saturday night. Leicester stood joint eighth wwith Derby Storm on 14 wins, but needed their East Midland rivals to lose at home to Sheffield Sharks.
After McGee's final basket, Birmingham's Fabulous Flournoy fluffed an attempted lay-up and Justin Phoenix clutched the rebound. The crowd were already mobbing the players and coaches when the game commentator, Daniel Routledge, announced: "We have the result from Moorways. Derby have lost: we're in the play-offs."
In a surprising final night in keeping with an unpredictable season, the deposed champions, London Towers, lost 97-81 at home to seventh-placed Newcastle Eagles last night, but finish runners-up and play them again in the play-offs.
Leicester, relieved of any pressure for their trip to the Sharks, lost 86-71 in Sheffield, but in another upset Birmingham lost again, beaten 98-95 in overtime by the unranked Worthing Bears. Sheffield finished third and play Manchester Giants in the play-offs while fourth placed Birmingham face fifth-placed Chester Jets.
The best-of-three series begin with London's visit to Newcastle on Friday.