The Sardinians enjoyed the better of the first half with their livewire striker Victor Ibarbo wasting the best chance of the game.
Struggling Siena reacted well in the second half, spending much of the 45 minutes on the attack, but they failed to test Michael Agazzi in the Cagliari goal as they were only able to move one point closer to Genoa in 17th place.
Cagliari, who have no such relegation concerns, appeared more confident in a tight first half.
Thiago Ribeiro headed wide before Ibarbo saw his shot blocked as chances were few and far between.
The enterprising Ibarbo then created a chance out of nothing, powering his way through two defenders, but when presented with only Gianluca Pegolo to beat, he lacked the necessary composure and struck the ball straight against the Siena goalkeeper.
Siena's response came in the second half when Reginaldo came on, giving the attack a lift.
However, they struggled to find a way through. Innocent Emeghera was as surprised as anybody to find the ball at his feet just six yards from goal and was unable to get any power behind a shot which appeared just to rebound off his foot.
Siena continued to attack in a one-sided second half, yet the best chance fell at the other end deep in stoppage time when Pegolo had to be alert to cut out a low cross which was begging to be tucked into an open goal by Nene.
Siena therefore missed the chance to move out of the bottom three.