Dean Cox and Chris Dagnall were the heroes for Leyton Orient as Russell Slade's O's booked their place at Wembley at the expense of Peterborough United at the Matchroom Stadium.
The first long throw of the evening from Scott Cuthbert caused all sorts of problems in the Posh back-line with goalkeeper Bobby Olejnik eventually pouncing on the loose ball with several players trying to force the ball over the line.
Experienced front man Kevin Lisbie sent an acrobatic overhead kick wide of the target on four minutes as Orient continued to dominate the early exchanges. Jack Payne saw a shot deflected wide at the other end after a move involving Swanson and Assombalonga.
On 11 minutes, Mooney fired a speculative volley wide of the upright after an intelligent knock down from strike partner Lisbie. Tommy Rowe tested Jones from 25-yards, but the keeper made a routine stop.
Bobby Olejnik made a superb stop on 35 minutes to push a skidding James' effort around the post after the midfielder was afforded time and space to set his sights.
Danny Swanson missed a glorious opportunity on 50 minutes to break the deadlock, but the Scotsman blazed wide of the near post with Assombalonga waiting for the ball across the box.
On 54 minutes, Posh were a whisker away from the breakthrough as Mark Little's effort skimmed the outside of the post after good work from Tommy Rowe.
Lisbie turned Knight-Percival on the edge of the area, but fired straight at Olejnik from 20-yards. On the hour mark, the hosts got themselves in front as COX was quickest to react to fire home after his free-kick had been blocked by the wall.
Little fired wide from the edge of the box after flicking the ball past Odubajo as Posh tried to get back on level terms. Ajose fired over from distance after finding a pocket of space 30-yards from goal.
Shaun Batt replaced Lisbie on 67 minutes as Russell Slade made his first change. A mistake from Knight-Percival allowed Mooney to nip in on 74 minutes, he squared for Batt whose shot went straight at Olejnik.
Posh made a double substitution on 75 minutes with Lloyd Isgrove and Conor Washington replacing Danny Swanson and Jack Payne.
On 89 minutes, the game was wrapped up as substitute Batt outpaced Alcock down the right and kept his head to pick out DAGNALL who swept home from six-yards to kill the tie.
In stoppage time, Posh did grab a goal back as Conor WASHINGTON fired home from just inside the box, but it mattered not as Orient held on for the victory.
Final Score: Leyton Orient 2-1 Posh (3-2 on aggregate)
Leyton Orient: Jones, Omozusi, Vincelot, Cuthbert, Baudry, Cox, James, Lisbie (sub Batt 67min), Mooney, Odubajo, Clarke. Unused subs: Sawyer, Larkins, Bartley, Dagnall, Lundstram, Simpson.
Posh: Olejnik, Little, Alcock, Knight-Percival (sub McCann 78min), Baldwin, Payne (sub Washington 75min), Swanson (sub Isgrove 75min), Rowe, Bostwick, Assombalonga, Ajose. Unused subs: Day ,Brisley, Ntlhe, McQuoid.
Attendance: 8,545 (1,416 Posh fans)