Vladimir Petkovic's men needed both a victory and fifth-placed Udinese to slip up at Inter Milan, but the Bianconeri did not oblige while Daniele Dessena claimed all three points for Cagliari with a late header.
Lazio now face Roma, who took sixth spot by beating Napoli, in the first-ever Rome derby final on May 26, with triumph promising the triple reward of a trophy, the bragging rights, and the chance to compete on the continent in the autumn.
Homeless Cagliari, meanwhile, ended their campaign at Trieste's Stadio Nereo Rocco but finished within touching distance of 10th-placed Parma at the end of an impressive season on the pitch.
It was a lethargic start in north-east Italy with Thiago Ribeiro waiting until the 12th minute to forced a punched save from Federico Marchetti following a corner.
Lazio hit back as Senad Lulic burst down the left wing to pick out Libor Kozak, who nodded the ball a yard wide of a post.
The 20th minute saw Cagliari go close to an opener as the Biancocelesti failed to clear their lines before Victor Ibarbo took a free shot against Marchetti's prone body.
Vlada Avramov made his first contribution in stopping Antonio Candreva's swerving drive before Ribeiro directed a fierce shot on the turn wide of Lazio's goal following good work from Ibarbo.
A disappointing half then drew to a close as Candreva opted to shoot at goal from a short free-kick with a host of team-mates lurking in the area.
Cagliari threatened following the restart, Sebastian Eriksson's pinpoint drive coming up against Giuseppe Biava and bouncing out for a corner.
Candreva tried again, shaping his body to volley with his right boot but meeting a block from Avramov, who repeated the trick on Lorik Cana's attempt from midfield.
With news that rivals for Europe Udinese were streaking towards a 5-2 win at Inter, Lazio's heads dropped and Cagliari took full advantage.
There were 75 minutes on the clock when Dessena tracked a free-kick from substitute Andrea Cossu to head the ball beyond a stranded Marchetti.
The decision was made to focus on next week's showpiece derby in the visiting camp and Ederson's tame free-kick from the edge of the box served as a white flag at the Nereo Rocco.