Cameroon's African Nations Championship campaign got off to a poor start as Congo-Brazzaville's Junior Makiesse struck a second-half penalty from which the Indomitable Lions never recovered.
Rigobert Song's Cameroon had enjoyed the better of the contest when goalkeeper Jean Oscar Gwot brought down substitute Vieljeux Prestige Mboungou in the 70th minute,
The Congolese attacker, breaking through on the counter, had initially taken the ball away from himself with a heavy touch. However, Gwot was also flummoxed and brought down the forward with an outstretched arm.
Makiesse duly opening the scoring from the spot, while Gwot was booked for persistently protesting his innocence, with replays appearing to show that he got a touch on the ball.
To that point, Cameroon had had by far the better of the contest, and will still be questioning how they weren't able to open the scoring just after the hour mark when - in a goalmouth scramble - both the woodwork and Congolese stopper Barel Mouko conspired to keep the scores level.
In the first half, Patrick Moukoko had blazed wildly over from six yards out after being picked out by a delightful ball from Frantz Pangop, while captain Junior Awono had also fired a thunderous effort over from 20 yards - another chance that the Lions were left to rue.
The win takes Congo up to top spot in Group D after Burkina Faso and Angola played out a goalless draw earlier in the day.
Angola and Cameroon meet in their second group clash in Agadir on Saturday, while Congo could seal progression when they face Burkina Faso.