Ebou Adams was sent off for a late challenge on Man of the Match Lamine Camara just before halftime.
Olympique Marseille midfielder Pape Gueye scored the opener after four minutes to give the Teranga Lions the lead in the SeneGambia derby.
The imposing midfielder latched on to Sadio Mane's pass before beating Baboucarr Gaye with a neat first-time effort.
The Gambia started brightly against their arch-rivals in the much talked about derby with Alasana Manneh forcing a save from Edouard Mendy as early as the second minute.
Musa Barrow and Ali Sowe came close for the tournament’s lowest ranked side but they couldn’t beat Mendy in the Senegalese goal.
Ismaila Sarr and Habib Diallo came close to doubling Senegal’s advantage only to be denied by Baboucarr Gaye.
Lamine Camara doubled Senegal’s lead with a brilliant finish from Ismaila Sarr’s pass six minutes after the restart.
Substitute Nicolas Jackson saw his goal bound effort saved by Gaye. The CAF Young Player of the Year Camara scored his second to wrap up an emphatic victory for Senegal.
The Gambia will return to Charles Konan Banny Stadium on Friday, 19 January to face Guinea at 20:00 GMT while Senegal will battle Cameroon at the same venue three hours earlier.