In a gripping encounter, Cameroon had looked comfortable after half an hour with Mbia's header and a fine free-kick from Geremi Njitap putting them two goals ahead. The score reflected their dominance but they rested on their laurels and were punished by a purposeful Tunisian side who took the game to extra time with a spirited comeback.
Chaouki Ben Saada curled in a fine free-kick of his own in the 34th minute and Tunisia might have levelled before the break when Brazilian-born striker Francileudo dos Santos rattled the crossbar, with Cameroon goalkeeper Carlos Kameni producing a fine save to deny Ben Saada on the rebound.
Cameroon sat back in the second half and found life increasingly difficult as their star striker Samuel Eto'o became isolated up front. Tunisia had the majority of possession in the second half and Yassine Chikaoui finally found the equalizer, driving home with his right foot past Kameni's despairing dive with ten minutes remaining.
Even then Cameroon might have gone back in front as Mohamadou Idrissou's left wing cross just evaded Jean-Joël Epalle's desperate lunge with the goal gaping.
The match went to extra time and Cameroon quickly put themselves back in front when Mbia volleyed home his second from Epalle's flicked header. Tunisia could only create half-chances from a succession of set-pieces and Cameroon eventually celebrated their win.
Cameroon will now face Ghana in Accra but might be without their star Eto'o who was guilty of an insane head-butt in extra time. The incident was not seen by the match officials but the Ghanaian media are sure to up the pressure on CAF to ban Eto'o from their semi final clash on Thursday.
In the earlier quarter final, a fluke goal from Amr Zaky gave Egypt a 2-1 win over Angola but again it is Manchester Utd's prodigal striker Manucho Gonçalves who grabbed the headlines with the best goal of the tournament so far.
After yet another Hosny Abd Rabo penalty had given Egypt the lead - his fourth goal of the tournament and third from the spot - Angola quickly responded when Manucho smacked the ball into the top corner of the Egyptian goal from over thirty yards; a strike that is sure to be replayed around the world as eager Manchester Utd fan's scour the net for footage of their new striker's extraordinary Cup of Nations exploits.
Manucho will not be able to add to his four goal tally, however, as Ahmed Fathy's right-wing cross struck Zaky in the stomach and flew into Angola's net. Manchester Utd fans must still wait to see their new buy at Old Trafford though, as he is set to move to Panathinaikos on loan for the rest of the season.
Manucho went close to equalizing after an hour but Egypt were composed in the latter stages and Emad Moteab and Fathy might have increased their lead.
Egypt now face tournament favourites Ivory Coast in Thursday's second semi final.
EGYPT 2 (Hosny pen 23', Zaky 38') ANGOLA 1 (Manucho 27')
TUNISIA 2 (Ben Saada 34', Chikhaoui 81') CAMEROON 3 (Mbia 18', 93', Geremi 27') a.e.t.
GHANA v CAMEROON (Thursday, 5.00 BST in Accra)
IVORY COAST v EGYPT (Thursday, 8.30 BST in Kumasi)
For full tournament statistics, go to http://news.bbc.co.uk/sport1/hi/football/africa/default.stm.