Northampton beat title rivals Exeter 18-8 to secure promotion back to the Premiership at the first attempt.
Jim Mallinder took over as director of rugby last June
The Saints' 26th successive win in National League One means second-placed Exeter cannot catch them, despite there still being four games left to play.
Hooker Dylan Hartley scored two tries and winger Paul Diggin one, while fly-half Bruce Reihana kicked a penalty in the match at Franklin's Gardens.
The sides will meet again in the final of the EDF National Trophy on 12 April.
Jim Mallinder's unbeaten side have scored 1,135 points and conceded just 295 in an impressive season following their relegation last year.
But Saints fly-half Carlos Spencer warned that next season will be a far more difficult proposition.
"Next year will be another level to what we've played this year," said the former All Black.
"The quality of ball we get in this competition will not be the same next year, because we will come up against a lot bigger forward packs.
"It is definitely a big step up from this division. We can't get too far ahead of ourselves, we've got to be realistic. We've got to look at the long-term goal."