James Ward-Prowse gave Southampton an early lead from the penalty spot after Long was bundled over in the area by Nampalys Mendy.
Shane Long scored his first goal for nine months as Southampton moved out of the Premier League relegation zone with a battling win at Leicester, despite spending the entire second half with 10 men.
Saints defender Yan Valery was then shown a second yellow card for a foul on Marc Albrighton, but Republic of Ireland striker Long doubled the lead just before half-time with his first Southampton goal in 22 appearances.
Wilfred Ndidi scrambled home Ricardo Pereira’s cross to bring Leicester back into the game at the start of the second half, but Southampton held on for a priceless victory.
The three points lift Southampton above Newcastle and Cardiff into 16th place. Leicester, meanwhile, drop one place to eighth.
It is a third win in seven Premier League games for new Saints manager Ralph Hasenhuttl, who punched the air with delight at the final whistle.
His side were forced to defend on the edge of their own penalty area for the entirety of the second half, but the Southampton backline was superbly marshalled by Jannik Vestergaard and Jack Stephens and the win gives them real hope of avoiding relegation.
The boos at the final whistle are unlikely to ease the pressure on Hasenhuttl’s Leicester counterpart Claude Puel, who had demanded a response from his side following last weekend’s FA Cup defeat at League Two Newport.
Instead his team were timid from the outset and despite dominating possession in the second half, rarely looked like threatening Alex McCarthy in the Southampton goal.
More to follow.