Benin international Sessegnon struck with a superb goal in the first-half and Gardner added an even better strike late in the second-half to move Sunderland into mid-table security and build on the much-improved form they have shown since Martin O'Neill replaced the sacked Steve Bruce in December.
It was the hosts first win over their opponents in eight matches since a 1-0 win in April 1964.
Sunderland made a vibrant start with Swedish striker Sebastian Larsson - making his 200th league appearance in England - almost giving them an early lead with a smartly-taken volley from Kieran Richardson's cross, but it came back off the post.
However, the Black Cats fans didn't have to wait too long for the deadlock to be broken as promising Irish wing James McClean found space and set up Sessegnon, who scored with a beauty of a shot to put them 1-0 up in the 14th minute.
The pendulum swung back the Welsh side's way shortly afterwards as the hosts lost Danish striker Nicklas Bendtner soon after with an eye injury and the Swans went close with a header by leading scorer Danny Graham.
Both sides were evenly-matched throughout the second-half but it was Sunderland who scored as Larsson's fellow former Birmingham team-mate Gardner struck late in the game with a stunning volley from Sessegnon's cross to seal the points.