An own goal from Kyle Owens and a header from Tommy Stewart sealed the win for the Inver Park side while Tony Kane got United’s only reply from the penalty spot.
Tiernan Lynch’s side are now through to the last four while United will try to lift themselves for the rest of the season.
It was also good to see a large Larne support at the Showgrounds as they looked to cheer their side through to the semi-final of the Irish Cup with money man Kenny Bruce from Purple Bricks among them.
The first 15 minutes was lively enough and the Inver Park men looked up to the challenge with Martin Donnelly doing well in the middle of the Larne midfield.
United’s Fra McCaffrey had the first real chance of the game in the 18th minute. His shot however was off target and the chance was lost.
Both sides were struggling to get a grip on the game as game lurched past 20 minutes.
On the half hour mark Larne counter attacked after Tony Kane gave the ball away. Jerry Thompson cross come shot was pushed away by the rushed Ross Glendinning in the United goal.
Three minutes later Ballymena were awarded a penalty after a Larne defender handed a shot from Johnny McMurray,
Tony McKane stepped up and it was 1-0 United as he sent Larne keeper Conor Devlin the wrong way.
Two minutes later it could have been 2-0. The ball dropped to McMurray in the box but he volleyed wide when well placed.
Larne will still very much in the game with five minutes left in the half but the front two of Tommy Stewart and David McDaid were being starved of the ball much to the frustration of the travelling support.
On 44th minute McMurray tried a volley from 25 yards - but his effort flew high and wide.
Six minutes into the second half it was 1-1 after a terrible mistake from United defender Kyle Owens.
Donnelly threw in a ball for Larne and Owens sliced it into his own goal past a stranded Glendinning. It was an awful attempted clearance from the big defender who had been doing well.
Two minutes Donnelly nearly gave the visitors the lead but he blazed over close range. This game was getting interesting.
United had another chance in the 59th minute but Conor McCloskey’s effort was cleared by Larne keeper Devlin.
The Sky Blues had another chance two minutes later. McMurray released McCaffrey who shot straight at keeper Devlin. Would United pay for these misses?
They did in the 67th minute as Tommy Stewart headed home from a lovely ball in from Donnelly to make it 2-1 and the Larne faithful went bananas.
United huffed and puffed and Owens had a header from a throw-in two minutes from time but Devlin gathered and Larne were through to the semi-finals, while the United players were booed opff by their own fans.
Ballymena United: Glendinning, Kane, Owens, McCloskey, Faulkner, Weir (McKinney, 77mins), Ervin, McCullough, Millar, McCaffrey (Shevlin 71mins) McMurray.
Subs not used: McDermott, Flynn,Balmer, Dobbin, Nolan.
Larne: Devlin, Henry, Thompson, Finnegan, Ramsey, McEleney, Caldwell, Scullion, Stewart (Tolan 90mins), McDaid, Donnelly.
Subs not used: Bradley, McCabe, Georgaklis, Kelly, McIvor.
Ref: A Davey