The Blucerchiati looked to make inroads in the opening minutes at the Stadio Luigi Ferraris, with Afriyie Acquah and Eto'o both having early strikes blocked by the Sardinian side's defence.
However, it was Sinisa Mihajlovic's men who broke the deadlock in Genoa when Luis Muriel's corner was met by Lorenzo De Silvestri in the heart of the area, and the defender's header proved too much for keeper Zeljko Brkic.
Early in the second half, Longo came close to a Cagliari equaliser when he reached Diego Farias's pinpoint cross, only for Viviano to save low down to his right.
Samp made sure of victory 18 minutes from time when Eto'o rifled Matias Silvestre's cross into the top corner of Brkic's net.
Mihajlovic's side held on for a victory that moves them level on points with Fiorentina, although the Viola have a superior goal difference.