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Long netted twice in the space of five minutes late in the second half at St Mary's and the Republic of Ireland star's first league goals for the club ensured the Saints will remain second in the table heading into the international break.
Despite a turbulent close-season marked by the departure of five key players and manager Mauricio Pochettino, Southampton have been in inspired form under new boss Ronald Koeman.
They have won eight of their last nine league matches and are starting to believe they could claim a shock place in the Champions League via a top-four finish.
Leicester proved obdurate opponents but Nigel Pearson's team couldn't hold out and they remain in the relegation zone after their sixth defeat of the season.
Southampton striker Graziano Pelle had the first chance when his effort was blocked from just inside the penalty area.
With torrential rain lashing down at St Mary's, quality play was at a premium from both sides in the first half.
Sadio Mane provided a rare moment of quality when the Senegal winger fired just over from a tight angle after his cross came back off Marcin Wasilewski.
Saints suffered a blow when midfielder Steven Davis limped off to be replaced by Jack Cork just before the break.
But Cork almost made an immediate impact when he turned on Victor Wanyama's pass, only to scuff his shot wide from the edge of the area.
Leicester had failed to score in four of their last five league matches and forwards Leonardo Ulloa and Jamie Vardy were both anonymous in the first half.
Pelle started the second half by lashing a first-time strike over the bar after good play from Mane down the left flank.
Moments later Mane turned in Ryan Bertrand's cross but the winger's effort was correctly ruled out for offside.
Pelle couldn't keep his shot on target and Bertrand brought a routine save from Kasper Schmeichel as the hosts pressed for the opening goal.
Koeman sent on Long in place of Mane in the 67th minute and it took just eight minutes for the switch to reap a rich reward.
Cork floated a pass into Pelle from the right and the Italian held off his marker long enough to tee up Long to bend a left-foot shot into the far corner from the edge of the area.
Long sealed the points in the 80th minute when he sprinted onto a long ball over the top of the Leicester defence, beating the offside trap to calmly slot past Schmeichel.
