Eto'o unleashed a brilliant first-time, 20-metre shot after defender Lucio misjudged a long ball to seal a game in which the second-string Brazilian team were below par.
Eto'o joined the African champions' squad only this week as he was involved with his club Mallorca in the Spanish league until last weekend.
But his great performance throughout the match confirmed coach Wilfried Schaefer was right to rush him straight into the action.
Thursday's setback was somewhat familiar for the five-time world champions, who won only once in the last edition of the eight-nation tournament two years ago in Japan and South Korea.
In mitigation, Alberto Carlos Parreira could point to the absence of Rivaldo, Ronaldo and Roberto Carlos.
But even with Emerson, Adriano and Ronaldinho - whose performance would not have impressed the interested Manchester United - they never looked able to overcome the strong physical challenge imposed by the Africans.
TIRELESS CAPTAIN
Brazil mastered the battle of the midfield in the first half thanks to tireless captain Emerson and striker Adriano, playing instead of Ronaldo, forced goalkeeper Idris Kameni to a good save in the 19th
Cameroon, banking on swift counter-attacks to break the solid Brazilian defence, had a good opportunity when midfielder Modeste Mbami saw his close-range header stopped by Dida just after the half-hour.
Backed by the most of the crowd, Cameroon started to believe in themselves and moved up a gear after the interval.
Dida was twice more called into action to foil Eto'o, the man of the match.
The 22-year-old striker eventually found a way through though as the Brazilian defence started to look tired and out of position in the latter stages.
A simple long clearance from a Dida goalkick soared over the head of Lucio and Eto'o lashed his shot past the keeper.
The striker received a standing ovation when he was replaced by Pius N'Diefi in injury-time.
The win leaves Cameroon second in the group B, level on points with Turkey, who they play in Paris on Saturday.
The Turks, beaten by Brazil in last year's World Cup semi-finals, came from behind to beat the United States 2-1 at St Etienne.
Turkey were down 1-0 after striker DaMarcus Beasley headed home a cross from Landon Donovan in the 37th minute but they equalised with an Okan Yilmaz penalty three minutes later.
Tuncay Sanli gave Turkey a deserved win in the 73rd minute.