Stags led the tie 2-1 after the first leg and McLaughlin ‘s strike on 32 minutes eased any nerves the visitors had.
They then defended brilliantly for 90 minutes and keeper Nathan Bishop's only real save came in the opening seconds from Northampton's first raid.
And the final whistle was the cue for celebrations as Stags basked in the glory of a forthcoming day out at Wembley.
Stags boss Nigel Clough said his side booking a Wembley final tonight was an incredible achievement.
He said: “Last time we were at Northampton we were absolutely down and out.
“We then put an incredible performance in on the Tuesday against Port Vale and picked up from there.
“To turn it round over seven or eight months and to be going to Wembley, I think the players and staff deserve a huge amount of credit.
“When you are joint bottom of the league after a third of the season, then to reach Wembley – and we still have a job to do there – is just an incredible achievement.