Goals from Miles Jones, Ryan Upward and Ryan Blake were enough to see the Evo-Stik League Southern Division One side through to the final, at the expense of an Alan Smith led Dons side who, despite dominating for large parts, weren't clinical enough as the non-Leaguers stole the show at a bitterly cold Holloways Park.
Former Manchester United and England striker Smith, who wasn’t able to play because of the red card he picked up on the weekend, named a strong starting XI which included the likes of Izale McLeod, Mathias Kouo-Doumbe, Antony Kay and Ryan Lowe.
Also involved were duo Tom Flanagan and George Baldock, who were making their first Dons appearances of the season. Youngsters Dele Alli and Brendan Galloway were also given run-outs, as well as goalkeeper Charlie Burns and defenders Mason Spence and Jon Osae.
However, despite the experience in the Dons’ starting XI, it was the non-League side who took the lead courtesy of winger Miles Jones, who hit a fierce shot across Burns from just inside the penalty area after latching onto a cross-field ball.
The Dons, who had been the better side in the opening half an hour almost hit back immediately through Kouo-Doumbe, however the Frenchman’s looping header had too much on it as it bounced off the crossbar and out for a goal-kick.
Beaconsfield threatened once again, ten minutes before half-time, but this time Burns was able to pull off a smart stop - getting a strong right hand to Gavin Blake’s low shot which allowed Kay to race back and clear.
Lowe could have pulled the Dons level five minutes later after he found himself with an inch of space on the edge of the visitors box, but the striker’s shot was straight at Rams ‘keeper Southorden who gathered with relative ease.
The crossbar once again came to Beaconsfield's rescue on the stroke half-time this time, though, it was Spence’s left-footed shot that crashed against the woodwork.
Soon after the re-start, captain for the night Flanagan had a good chance to equalise when he climbed highest to meet an in-swinging corner, however the defender didn’t get enough on the ball as it flashed wide of the post.
Smith’s men spent the first 15 minutes of the second period camped in the Rams half and the constant Dons pressure looked to have paid off 20 minutes into the second half when Galloway lined up a shot 30-yards from goal, however a superb acrobatic save from the Rams' goalie prevented him from piercing the net with a stinging effort.
Southorden was then forced into making another important stop moments later after Baldock found himself deep inside the Beaconsfield box - the lanky shot-stopper getting fingertips to the young midfielder’s flick which looked destined for the bottom corner.
The home team, though, stunned the men from MK1 with 15 minutes left on the clock when Ryan Blake found room to work on the right flank. The pacy forward, who had been limited to touches on the ball for most of the game, was able to drive into the penalty area and drill in a low cross which was met at the far post by Ryan Upward who couldn’t miss from 2-yards.
The tie was then sealed five minutes from time when Blake was able to get a shot away despite heavy pressure from several Dons defenders - his right-footed effort beating an un-sighted Burns from just inside the box to knock the League One Dons out of the county competition.
MK Dons (4-2-3-1): Burns; Flanagan (c), Kouo-Doumbe, Kay, Osae; Galloway, Baldock; Spence (Summerfield 46), Alli, Lowe (Hickford 77); McLeod.
Subs not used: Grant, Ahmed, Forrester.