Jim Mallinder hailed Saints for holding their nerve after they saw off Connacht at Franklin's Gardens.
Mallinder's men had flanker Jamie Gibson sin-binned two minutes from time, but shut their opponents out to earn a 21-15 win.
It means Saints will now host Stade Français in the play-off final on Friday night, with the winner of that match taking a place in next season's Champions Cup.
Saints had suffered late heartache in several Aviva Premiership matches towards the end of the campaign, but it wasn't to be a similar story against Connacht.
Courtney Lawes made a huge hit to turn the ball over, bringing an end to the game to ensure it will be Saints who host Stade next weekend.
And Mallinder said: "Because we've lost a number of games towards the end, it was nice to win one.
"Our defence was good throughout the game, we had some good linespeed, made good tackles and showed good composure towards the end."
Saints' season would already have been over had Stade lost to Gloucester in the Challenge Cup final.
But the French side did the business in Edinburgh and after Mallinder's men defeated Connacht, one more win will be enough to earn Champions Cup rugby for next season.
"It was remarkably easy to (motivate the players)," Mallinder said.
"I was worried about this week and peoples' minds thinking they should be on holiday, with the Lions or with England, but once they came back Monday the attitude's been very good and you saw that today.
"It wasn't 100 per cent accurate, but there were some good things in there and defensively we were pretty good."