Mbia's header in the dying stages at the Mestalla earned Sevilla a 3-3 aggregate scoreline, with Unai Emery's men progressing on away goals - with the Cameroonian's injury-time effort the only goal scored away from home in the two legs.
Sevilla had coughed up the 2-0 advantage they had opened up in the first leg, as Valencia took the lead inside the final 20 minutes in front of their raucous home fans.
But Sevilla still knew just one goal would be enough for them to advance, when Federico Fazio's flicked-on header found Mbia in the box, who did the rest.
"This goal is for my family, for my mother and my father. I'm so emotional. This is the happiest moment of my life," Mbia said.
"It was important for us to get to Turin, after we'd worked so hard to get there.
"I feel so happy because the tie had become so hard of us and it was difficult to play here. We've very happy and the goal was incredible."
Sevilla attacker Ivan Rakitic said he thought the tie was over when Jeremy Mathieu netted in the 70th minute to put Valencia 3-2 up.
"None of us have the words to explain what's happened. It's incredible. We work hard every day, we deserve this and so do our fans," Rakitic said.
"I'd be lying if I said we didn't think we were out of the final, when they scored their third. We kept going right to the end though. The goal from Mbia was wonderful."
Rakitic dedicated the win to Sevilla's past European cup winners, with the La Liga side winning the second-tier continental tournament in 2005-06 and 2006-07.
"There should be a monument to all of them. I remember the goals of (Antonio) Puerta, the saves of (Andres) Palop and now this.
"We've no need to think about Benfica right now. We'll enjoy the moment. When we get there, we want to win it."
