The £12 million summer signing came off the bench to score twice in the last 15 minutes as Ronald Koeman's dream start to life at St Mary's showed no signs of letting up
Second half substitute Shane Long, a £12 million summer signing, scored twice in the second half, breaking the deadlock with a well-taken finish before adding a second after combining with Victor Wanyama.
The win keeps Ronald Koeman's side in second place, four points behind Premier League leaders Chelsea, while Leicester's slide down the table continues.
Saido Mane was restored to the Southampton starting line-up at the expense of Long in the only change from the team that beat Hull City last weekend.
Leading scorer Leonardo Ulloa returned to the Leicester side along with Jeffrey Schlupp, as David Nugent and Paul Konchesky dropped to the bench.
Koeman had urged his in-form side to dominate Leicester and they wasted no time in getting on the front foot.
Ryan Bertrand proved to be a dangerous outlet and Schlupp had to be alert to head the on-loan left-back's dangerous cross behind for a corner at the back post 20 minutes in.
Leicester were unable to keep hold of the ball as Southampton continued to pin them back, but Mane typified the hosts' lack of composure in the final third when he fired a shot over the crossbar from inside the penalty area.
Koeman was forced to make a change five minutes before the break, with Jack Cork replacing the injured Steven Davis.
Mane thought he had put Southampton in front seven minutes into the second half when he turned in a Bertrand cross, but the Senegal winger had strayed offside.
Leicester stood firm as the home side continued to dominate without being able to come up with an end product.
The visitors were living dangerously, though, and Morgan Schneiderlin's shot was deflected wide for a corner after Dusan Tadic's cross had fallen nicely for the midfielder in the penalty area.
And Southampton finally got the reward for an another impressive display when Graziano Pelle laid the ball off for the onrushing Long - on in the 68th-minute - and he kept his composure to finish with his left foot.
The Republic of Ireland international struck again 10 minutes from time to seal the points, beating the offside trap before calmly slotting beyond Kasper Schmeichel.
Chris Wood ought to have pulled a goal back three minutes from time, but he was somehow off target with only Fraser Forster to beat.
Long almost had a hat-trick with the last kick of the game when he fired just wide after a swift break, but his double ensured Southampton's sensational start to the season continues.