Sonny Carey and Lyndon Dykes netted first-half goals for Charlton as they made it a 2-0 losing start for new Leicester interim boss Andy King at the King Power Stadium.
Foxes defender Caleb Okoli was sent off early on - the Italian was the last man when he fouled Miles Leaburn - and Jordan Ayew failed to convert a penalty in the second half to cap off a miserable afternoon for the hosts.
Leicester City owner Khun Aiyawatt 'Top' Srivaddhanaprabha was in attendance for King's first match at the helm after the sacking of Marti Cifuentes on Sunday.
But the Foxes faced an uphill task from the moment centre-back Okoli was dismissed in the 15th minute.
Leaburn looked set to race through on goal from Carey's flick but Okoli pulled him back - referee Andrew Kitchen brandishing a straight red.
Charlton controversially saw a 21st-minute goal cancelled out when Carey's corner was headed in by Harry Clarke, even though there seemed to be no infringement on goalkeeper Jakub Stolarczyk.
The Addicks went ahead in the 36th minute as Carey bagged his seventh goal of the campaign. The midfielder controlled Clarke's low cross from the right to tuck the ball inside Stolarczyk's left-hand post.
Leicester's keeper saved with his left foot to deny Liverpool loanee Luke Chambers as the match moved into 10 additional minutes of stoppage time due to injuries to players on both sides.
Charlton extended their advantage before the break as Dykes steered Chambers' cross back across Stolarczyk - the Scotland international's third goal this season and first for Charlton.
Leaburn had proved a problem for the City backline but did not reappear at the start of the second half as he was replaced by Matty Godden, who saw a smart turn and shot deflected over by Ben Nelson.
Leicester failed to have a shot on target in the first period but made a far more positive start to the second half.
Veteran striker Ayew spurned a huge opportunity to cut the deficit. But his penalty, awarded after Abdul Fatawu's arm was tugged by Amari'i Bell, hit the left upright.
Charlton goalkeeper Thomas Kaminski was called into his first save after 58 minutes as he went full stretch to keep out Ricardo Pereira's low attempt from the edge of the box with the resulting rebound scrambled out for a corner.
There were big cheers from the home support when Ayew was replaced by Patson Daka in the 76th minute.
Leicester's Louis Page flashed a right-footed shot over Kaminski's bar in the closing stages but there would be no comeback.
Leicester's interim boss Andy King:
I don't think we can get away from the fact we're not where we want to be in the table. But let's try looking up and reversing the momentum, rather than looking down.
"My remit was to do the Charlton game. I've not looked too far past that.
"You work hard all week and the game plan is ripped up after 15 minutes. Then you lose another senior player when you are short of bodies.
"We said at half-time to the lads that this could go one of two ways. You either go under and feel sorry for yourselves. Or can we try and make it work, even though we're down to 10 men. I was pleased with the response.
"It's a red card. I've played and it is difficult in those moments - it is a split-second decision. Caleb will learn from it. He is missing one game and then we will have to move on. He can't feel sorry for himself. We need to park that, learn from it and come back more hungry to help the team.
"The penalty was a key moment and it was always going to be difficult after that.
"The reality of football is you need to win games but it was unfortunate with the red and losing Hamza early on. We were really, really light. Harry Winks was excellent when he came on."
Charlton's Nathan Jones:
"We scored three goals of the highest quality because I have absolutely no idea why the corner is disallowed. His own player blocked him and he (referee Andrew Kitchen) absolutely guessed. There is no foul there. There is no-one around the goalkeeper.
"It is a massive win. The result was everything today. We wanted to be front-footed and we went two up top. We wanted to be ultra-aggressive against them and ultimately that changed the game.
"It is a definite red card. If he didn't do it then Miles is one on one against the goalkeeper. He is clean in.
"It is a wonderful finish by Sonny. There is a lot of development to come from him because defensively he has got to be better. He gets in those situations and has been excellent for us.
"Last week (the 4-0 loss at Millwall) was really hurtful for the football club. I wanted to have a reaction. We're going to have tough days this season because it is a monster of a league."
