Seemingly with it all to do after Thursday’s 2-0 home defeat, Keith Curle’s Cobblers scored three times without reply at the Jonny-Rocks Stadium to clinch a meeting with Exeter City in the behind-closed-doors final at Wembley on 29 June.
Cheltenham’s solid-looking first-leg advantage was halved within nine minutes as Northampton were rewarded for a bright start through Vadaine Oliver’s clinical downward header.
On-loan West Bromwich Albion forward Callum Morton then scored twice in the second half to complete a deserved turnaround for the spirited visitors, with the winner coming 13 minutes from time as he capitalised on a disastrous mix-up in the hosts’ defence.
Monday’s dramatic comeback kept alive Northampton’s hopes of returning to League One after just a two-year absence, while Cheltenham now face a 12th consecutive season below the third tier.
Morton was mobbed by his team-mates after netting the winner, with any thoughts of socially distanced celebrations momentarily forgotten by the jubilant Northampton players, who fully merited their victory.
But the decisive goal came in fortunate circumstances after an uncharacteristic moment of calamity at the back from the hosts, who had only conceded 27 league goals all season before Monday’s drama.
Only Premier League leaders Liverpool could boast a statistically meaner defence than the Robins in England’s top four leagues this term, but it was hard to fathom that as Northampton snatched their third of the night.
Cheltenham defender William Boyle attempted to allow a stray ball to roll back to his goalkeeper Owen Evans – who had saved a penalty in a brilliant first-leg display between the sticks – but the pair misjudged the situation completely.
The on-rushing Michael Harriman duly pounced and his shot struck the base of the post with Morton able to slot in on the rebound.
That came after the 20-year-old had added to Oliver’s first-half opener, as more slack Cheltenham defending saw the ball hit Morton’s head after Charlie Goode’s initial header bounced around the box.
After Morton’s second, Alfie May twice went close to levelling the tie on aggregate in the closing moments, but Cheltenham failed to force extra time.
It was a frustrating end to an otherwise excellent first full campaign in charge for former Northern Ireland, Burnley and Cheltenham defender Michael Duff, who took over as manager in September 2018 when the Robins were 18th in the table.
Meanwhile, Curle’s side had lost seven of their previous nine games in all competitions but are now just one win away from promotion.
With their fightback, they became the first team in the English Football League to lose the first leg of a play-off tie by two or more goals and then progress to the final since Yeovil Town fought back against Nottingham Forest in League One in 2007.