The gangling striker struck the second and third Senegalese goals at a packed Stade Alphonse Massamba Debat as they booked a place in the final against Nigeria or Burkina Faso.
But the tall shot-stopper had no chance after 16 minutes when an acrobatic overhead kick from Cheikhou Dieng looped into the net.
The combination of a defence-splitting pass and a timely off-the-ball run by Keita set him up to fire low past Mafoumbi on 37 minutes for a 2-0 half-time lead.
Congo improved dramatically after the break and Merveil Ndockyt reduced arrears on 55 minutes via a close-range shot after a cross was not cleared.
Both sides had chances in an entertaining game before Keita put the outcome beyond doubt nine minutes from time with a header into the corner off a cross.
It was the third goal of the tournament for Keita from Senegalese champions AS Douanes, who scored in a draw with Nigeria last Saturday.