Diego Simeone picked Torres from the start for only the fifth time in LaLiga this season and that decision paid off, as the Spaniard put in a strong performance leading the line and gave Leganes problems throughout.
Torres made his Atletico debut against Leganes in May 2001 so it was fitting that the fans' favourite was crucial for the opening goal – converting the rebound after Antoine Griezmann missed a penalty won by the former Liverpool star.
Proceedings turned somewhat tepid for the remainder of the first period, but Atletico returned after the break with renewed focus and doubled their lead shortly after, with Torres again silencing the visitors.
Leganes rallied and did craft a couple of good chances in the final 30 minutes, though Atletico were able to see the win out with minimal fuss, ending a four-match winless run across all competitions and closing to within three points of third-place Sevilla ahead of their hosting of Villarreal on Sunday.
Atletico's first chance fell to the 32-year-old in the second minute, as he raced on to Griezmann's pass over Leganes' high defence and sent an audacious chip just wide of the left-hand post.
Torres was decisive upon his next venture forward, though, as he lured Dimitrios Sivas into a foul in the area with a clever touch and then stabbed home from close range after Griezmann's spot-kick was kept out by Iago Herrerin.
Atletico struggled to build on that goal and Leganes looked more comfortable as the half progressed.
But they had trouble testing Atletico goalkeeper Miguel Angel Moya, with Victor Diaz's 20-yard drive into the side-netting just before the break the closest they went.
There was a better tempo to proceedings at the start of the second half and Atletico made the most of that by doubling their lead after 51 minutes.
Half-time substitute Angel Correa had made a difference and he was at the centre of things for the goal, feeding Torres into the box with a fine pass and the striker applied a lovely, dinked finish beyond the approaching Herrerin.
Leganes went close to pulling one back twice in the 68th minute - debutant Alberto Bueno first seeing his 16-yard effort deflected just wide by Diego Godin, before Diaz diverted the resulting corner towards goal and forced Moya into a good save down to his left.
Correa almost scored the brilliant goal that his second-half display deserved when seeing a stinging left-footed drive go just wide, but that miss mattered not, as Atletico comfortably saw things out.