Robert Huth's own goal and a Jermain Defoe effort put Sunderland in control in the second half.
Robert Huth's own goal and a Jermain Defoe effort put Sunderland in control in the second half and Shinji Okazaki's strike came too late to prevent a fifth defeat in nine league games for Leicester.
Claudio Ranieri's side lie just two points above the bottom three, while Sunderland continued their steady improvement under David Moyes by recording a third victory in four games to move into 18th place.
Jamie Vardy's Premier League goal drought now stretches to three months on another day to forget for Leicester, who fell behind in the 64th minute thanks to the kind of misfortune to which they were largely immune en route to lifting the title in May.
From Duncan Watmore's corner, substitute Jan Kirchhoff's off target header was going nowhere until it struck Huth to find the bottom corner of Ron-Robert Zieler's net.
After Patrick van Aanholt's penalty claims were turned down when the Sunderland defender appeared to be tripped inside the area by Danny Simpson, in what was a chastening afternoon for the former Black Cats full-back, Defoe doubled the Wearsiders' advantage with 13 minutes remaining.
Watmore was again the line of supply, when his shot from a Victor Anichebe cross was blocked, as the ball fell invitingly for Defoe to fire home into the corner from close range for his eighth league goal of the season.
Leicester showed the kind of character for which they became synonymous last season as they halved the deficit two minutes later, Japan striker Okazaki stealing in at the near post ahead of Papy Djilobodji to poke home a cross from Demarai Gray.
Jordan Pickford saved brilliantly from Wes Morgan at a stoppage-time corner as Leicester pressed in vain for an equaliser.
They could have few complaints at the outcome after enduring an early storm where they could have found themselves two goals down.
Nigeria forward Anichebe thrived down Sunderland's left and saw a goal-bound effort blocked by Morgan inside the six-yard box.
Anichebe had justifiable penalty appeals turned down when he appeared to be held back by the busy Leicester skipper as his close-range shot was blocked by Huth after the Foxes failed to deal with Didier Ndong's centre into the area.
Anichebe created another chance soon after, using his pace and power to beat Simpson and centre for Defoe, who uncharacteristically scuffed his shot from 10 yards.
Leicester eventually got to grips with Ndong in Sunderland's midfield as the visitors began to gain a foothold.
Vardy threatened three times with headers in the 20 minutes before the interval.
The best of these openings saw the Leicester forward lose his marker to meet Marc Albrighton's cross to send a looping header over Pickford's goal.
Christian Fuchs, who provided a telling centre for Vardy to send a glancing header narrowly wide, ended the first period by seeing his cross from the left bounce off the Sunderland bar.
Islam Slimani saw his snap shot early in the second half strike the hand of Djilobodji in the area, but his penalty appeals proved fruitless as, boosted by the introduction of experienced midfield pair Kirchhoff and Seb Larsson from the bench, Sunderland quickly gained the upper hand.
They translated that superiority into two priceless goals, the second of which by Defoe, the 151st of his career, saw the former Tottenham forward move above former England international Michael Owen into seventh on the all-time Premier League goalscorer's list.