America came from behind to beat Toluca 3-2 on aggregate after their 2-0 home leg win.
Goals from Aquivaldo Mosquera and Jesus Molina - both headers from crosses - saw Miguel Herrera's men book their final spot.
Herrera's time at the club is coming to an end after he was given the reins to lead the Mexico national team to the World Cup in 2014 and beyond, but his chargers extended his finale to the two-leg decider starting Wednesday.
Toluca would have been disappointed to end their season on a loss, having carried a lead from their home leg, and salt into their wounds was an 88th-minute dismissal to Sinha.
Leon wrapped up a 5-3 aggregate victory over Santos Laguna in their two-leg tie, with the finalists earning a 2-2 road draw.
Oribe Peralta breathed life into the clash at the Estadio Nuevo Corona, giving the hosts hope of overturning their 3-1 deficit from the first leg with a 51st-minute goal.
However, those hopes were short-lived - as Mauro Boselli responded for the visitors five minutes later, much to the joy of manager Gustavo Matosas.
When Carlos Pena struck on 89 minutes after rounding goalkeeper Oswaldo Sanchez, the tie was over.
A penalty for Santos Laguna on the whistle – converted by Juan Rodriguez – extended their home unbeaten streak to six matches, but it was scant consolation as Leon celebrated reaching their first final since 1997.
Leon are also aiming to break a title drought that extends back to 1991-92.
America, though, broke through to win the Clausura title earlier this year, and can be the first side to do the double in the same calendar year since Pumas UNAM in 2004.
