After Benjamin Mbunga-Kimpioka and Luke Molyneux put Sunderland ahead, Sam Hughes narrowed the arrears before half-time, but Molyneux added another following Jack Diamond's solo strike.
After beating Wolves to reach the Premier League Cup semi-finals last week, Eldin Jakupović came in for Daniel Iversen while Liandro Martis was replaced by Josh Eppiah in the North East.
Leicester suffered an alarming start as Sunderland went ahead shortly after the first whistle when Diamond crossed for Mbunga-Kimpioka to spin and drive it low into the corner.
Their lead was doubled on 20 minutes, too, as Molyneux cut inside his man on the edge of the area before picking his spot into the bottom right corner of Jakupović’s goal.
The frustration was evident on the faces of Leicester’s young players and they took measures to try and half the deficit as quickly as possible.
First Josh Knight, playing in a more advanced midfield role, broke free of his man and laid it up for Eppiah in the centre, but the Belgian forward's effort drifted wide.
City's pressure did pay off in the 26th minute when Hughes, who turned 21 on Sunday, capitalised on a spill from 'keeper Max Stryjek to get one back with his 11th goal of the 2017/18 campaign.
However, a succession of spurned opportunities, from Harvey Barnes, George Thomas and Kiernan Dewsbury-Hall, ultimately cost them as Sunderland extended their advantage.
A lovely run and finish from Diamond made it 3-1 before Molyneux added his second of the afternoon by rounding Jakupović and finishing tidily into an empty net.
As a result, despite a valiant effort, the Foxes were forced to stomach defeat by a Sunderland side intent on persevering their Division 1 status.
Major moment…
Sunderland’s third was a cruel blow for City in the North East and it came at a point when the visitors looked likely to equalise ahead of the final half-an-hour.
While Leicester will view the opening 20 minutes as a defining factor of just their fifth reverse this season, it was Jack Diamond’s effort which ultimately put the match beyond their reach.
Where does it leave us?
City remain second in Premier League 2 Division 1, three points behind Arsenal, who play West Ham United on Friday, and two above Liverpool, who host Chelsea next Sunday.
Coming up…
Leicester have a chance to recover on Monday 23 April when Derby County visit King Power Stadium on the final day of the 2017/18 Premier League 2 season.
The details…
Sunderland: Stryjek, Gamble, Hume, Bale (Hickey 88), Taylor, Storey, Diamond (Shields 77), Embleton, Molyneux, Mumba, Mbunga-Kimpioka (Connelly 84)
Subs not used: Talbot, Wright
Goals: Mbunga-Kimpioka 1, Molyneux 20, 73, Diamond 62
City: Jakupović, Johnson, Knight, Pascanu (Wright 45), Wood, Hughes (c), Dewsbury-Hall (Ndukwu 67), Thomas, Barnes, Gordon (Martis 72), Eppiah
Subs not used: Iversen
Goal: Hughes 26
Referee: Mr G Hart
Images: Sunderland Echo