First-half goals from Danilo Orsi and Klaidi Lolos put Crawley into the League Two play-offs with a 2-0 home victory over Grimsby.
The Red Devils sealed a seventh-placed finish with a first win in five games to set up a two-legged semi-final with MK Dons.
Crawley finished the regular season with 21 wins, 10 more than when they narrowly avoided relegation last term.
Crawley boss Scott Lindsey described his side’s season-defining clash as the most important of his managerial career, coming a year after he saved the club from relegation.
The hosts enjoyed a big let-off after only nine minutes when the unmarked Cameron Gardner blazed over from six yards after a cross by Denver Hume.
Mariners skipper Kieran Green also put a low header wastefully wide before top-scorer Orsi put Crawley ahead with his 21st goal of the season.
The former Grimsby striker poked the ball home from close range after 24 minutes following a cross to the back post by Jeremy Kelly.
Lolos doubled the advantage 12 minutes later when he fired into the corner past goalkeeper Jake Eastwood for his fourth goal in five games.
Grimsby threatened shortly after the interval when Hume shot wastefully over from a good position.
Neither side created anything of note in the second half and at the final whistle news of draws for both Barrow and Crewe triggered a pitch invasion at the Broadfield Stadium as Crawley celebrated reaching the play-offs for the first time as a league club.
