Long’s eighth goal of the season put the hosts ahead on 17 minutes with a fine header from a brilliant Matt Targett cross for a 1-0 lead that they kept until half-time, and deservedly so as they restricted a lively Watford side throughout the match.
The game was made safe when Tadić, on as a substitute, scored within two minutes of his introduction to the contest. He settled matters just before the final quarter of an hour with a clinical strike after some hesitancy at the back allowed him through on goal.
An encouraging performance and result was further good news for Saints on a night when Fraser Forster marked his return to action with a clean sheet which was testament to the Ronald Koeman’s side’s endeavour on the night.
Saints made three changes to the team that started the defeat to Crystal Palace in the FA Cup with goalkeeper Fraser Forster the headline change as he came in for his first senior appearance of the season.
Full-backs Cédric and Ryan Bertrand also returned to the team, with the latter playing further on the left side of a back three alongside Virgil van Dijk and skipper José Fonte.
It was a quiet start to the contest, though both Targett and Cédric each displayed good intent with sprints at either end of the field to keep play alive and then prevent a dangerous looking break from the visitors.
The contest soon became more eventful however, with the hosts’ patient approach probing a Watford side seemingly uncomfortable with the pressing from wide areas on a surface made slicker by the wet weather.
Saints broke the deadlock in the 17th minute of the game with a well-worked move that saw Targett’s fantastic searching cross flicked home by the head of Shane Long into the bottom right corner for 1-0.
The goal gave Koeman’s men a strong foothold in the game and they certainly had good opportunities to extend their lead before half-time.
The pick of those chances came on 21 minutes when Targett set Sadio Mané away on goal and the Senegalese was patient enough to beat two defenders before getting a shot on target which drew a superb one handed save from Heurelho Gomes who denied a certain second goal.
Saints continued to look confident going forward with Long attempting a lob from an acute angle which Gomes clawed away while Targett also unleashed a fierce strike wide.
At the other end, Ryan Bertrand produced a fine block to deny a relatively quiet Odion Ighalo who had got the better of Fonte down the right side of the area before pulling the trigger from ten yards out.
Nine minutes before the break Mané had another attempt on goal when he showed great awareness to beats Prödl to a loose ball from Jurado’s weak pass, but the frontman was faced with Goems on the edge of the area and he saw his low shot scuff off the left post and go wide.
A 1-0 lead at the break for the hosts meant Watford were behind in a game for only the second time at this stage all season.
Saints made a solid start to the second 45 and almost doubled their lead when Mané again caused the Watford backline problems, before he went up for a header that saw him collide with Gomes after his goalbound effort was cleared.
Gomes stayed down for a couple of minutes following the clash, but was deemed OK to carry on.
His opposite number, Forster, had been largely untroubled but received a loud cheer when he caught Étienne Capoue’s deep cross just past the hour mark, while he also continued to kick long when required.
Saints remained the more likely looking team to score again, and they had the ball in the net on 65 minutes when Romeu delivered a fine ball forward and Davis produced a smart finish but the flag for offside had already been raised long before the ball found the back of the net.
The hosts went close again when Van Dijk nodded the ball back across goal from a deep cross, and while Long was inches away from connecting to a tap-in, Mané ended up putting the ball into the side netting.
On 71 Mané struck another effort wide and that would be his last kick of the game as he was replaced by Dušan Tadić.
The Serbian delivered a near instant impact as he pounced on some uncertainty in the Watford defence, before keeping his composure to slot the ball under Gomes for 2-0 with virtually his first touch of the game.
It was nearly three when Tadić inspired another good break forward which saw Steven Davis unselfishly tease a cross towards Long, who couldn’t quite divert the ball goalwards under pressure from a last ditch challenge.
The remaining substitutions saw Graziano Pellè make his return from a knee injury in the closing stages of a near perfect evening for Saints, who got secured their second successive home league win in convincing fashion.
Southampton team (3-5-2): Fraser Forster; Virgil van Dijk, José Fonte (c), Ryan Bertrand; Cédric (Maya Yoshida 81), Steven Davis (Graziano Pellè 81), Oriol Romeu, Jordy Clasie, Matt Targett; Sadio Mané (Dušan Tadić, 71), Shane Long. Unused substitutes: Kelvin Davis (GK), Cuco Martina, James Ward-Prowse, Juanmi
Booked: Cédric (24)
Goals: Long (17), Tadić (73)
Watford team (4-4-2): Heurelho Gomes; Allan Nyom (Victor Ibarbo 68), Miguel Britos (Craig Cathcart 72), Sebastian Prödl, José Holebas; José Jurado, Ben Watson, Étienne Cappoue, Almen Abdi (Ikechi Anya 61); Troy Deeney, Odion Ighalo. Unused substitutes: Giedrius Arlauskis (GK), Valon Berhrami, Juan Carlos Paredes, Adlène Guédioura
Booked: Prödl (67), Deeney (77)
Referee: Craig Pawson
Attendance: 28,399 (including 2,470 away)