Ronald Koeman's side dominated a one-sided Premier League clash on the English south coast on Saturday, but they had to wait until 15 minutes from time for Long to open the scoring after coming off the bench.
Leicester had produced a resolute display to frustrate the hosts, but their resistance was ended when Long scored his first top-flight goal for the club with a clinical finish just seven minutes after coming on.
Long then sealed the points five minutes later as second-placed Southampton claimed an eighth Premier League win in their last nine matches to ensure they head into the international break four points adrift of leaders Chelsea.
Leicester, by contrast, are in the relegation zone having picked up just one point from their last six matches since their dramatic 5-3 win over Manchester United. They have also failed to score in four matches.
Sadio 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.
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