On his sixth appearance since joining from Everton in January, the former Cameroon international opened his account with a thumping drive in the second half.
Samp were already one goal to the good by then, after Lorenzo De Silvestri had met Luis Muriel's right-wing corner with a powerful header in the 33rd minute.
With 18 minutes remaining, Eto'o chested down Matias Silvestre's pass and smashed in a volley, before taking a camera from a nearby photographer and pretending to snap pictures of the crowd.
Cagliari's faint hopes of getting anything from the game disappeared eight minutes from time when Danilo Avelar was shown a second caution for bringing down Eder.
The visitors remain in the relegation zone, with just one point from their last six matches, while Samp put the pressure on their rivals in the race for a top-three finish.
