Goal-machine Oribe Peralta netted just past the hour-mark to hand three points to the Mexicans, who recorded a first ever victory over an African side at the World Cup finals.
There was a party atmosphere in the stadium but fans were left to dance in the rain as the torrential downpour made conditions difficult for the players. They were fortunate to be playing on an immaculate surface that held up remarkably well to the conditions.
The first half was more about goals that weren't scored than those that did find the back of the net.
Three times the ball crossed the line, twice for Mexico, but on each occasion the assistant referee's flag went up.
First Mexico's Giovani dos Santos saw his volleyed effort disallowed when replays showed he was in line with the last defender, before at the other end Eric Maxim Choupo-Moting's disallowed goal was more clear-cut.
The unlucky Dos Santos was denied again later in the half when his seemingly good header at the back post from a corner was chalked off, even though the last touch appeared to come off Choupo-Moting.
Other than that, opportunities were limited. Samuel Eto'o flashed a shot wide of goal after excellent wing-play from left back Benoit Assou-Ekotto to set him up. It was a rare wild swing from the veteran forward.
But, somehow, the score remained level at the break.
Cameroon made an change at halftime, replacing their only home-based player in the squad, Cédric Djeugoue, who had been exposed on a couple of occasions at right back. In for him came Besiktas player Dany Nounkeu.
Mexico should have had the lead inside two minutes of the second period.
Dos Santos' beautiful touch into the path of Oribe Peralta put the Mexico striker free on goal, but Cameroon goalkeeper Charles Itandje raced off his line to make the block.
The goal did arrive for Mexico just past the hour-mark though as their pressure finally paid dividends.
Héctor Herrera's pass set Dos Santos free and when his shot was blocked by Itandje, master poacher Peralta was on hand to turn the ball home.
The Indomitable Lions could create little in the way of goal-scoring opportunities for the remainder of the match to register a disappointing start with bigger challenges to come.
Cameroon have now won just one of their last 14 games at the World Cup finals since the 1990 tournament in Italy made them many a neutral's favourite.
Next up for the African side is a meeting with Croatia on Wednesday in Manaus at 22h00 GMT.