Simone Padoin's first-half goal ensured that Atalanta's good opening day record continues as they edged out a Sienna side who had Massimo Maccarone sent off in the dying seconds to complete a miserable day.
Padoin gave the hosts the lead in the 15th minute and Siena never really troubled the Atalanta defence.
Any hope the visitors had of taking a point back to Tuscany disappeared four minutes from time when Maccarone was sent off for a second bookable offence.
The former Middlesbrough forward could find himself in even more hot water after he pushed the referee when he flashed him the red card.
Siena coach Marco Giampaolo was eager to get his Bianconeri reign off to a good start after taking over from Mario Beretta this summer.
The former Cagliari boss will be dismayed by this result and his side's display will give more force to his argument that he needs reinforcements before the transfer window closes at midnight on Monday.
Atalanta have not lost the opening fixture at home for 23 years and they were eager to preserve that proud record.
They took a deserved lead in the 15th minute when Cristiano Doni played in Ferreira Pinto, who sent a perfect cross into the box which Padoin headed past new boy Gianluca Curci in the Siena goal.
The hosts missed a great opportunity eight minutes later to double their lead as first Pinto's effort was blocked by the Siena defence before Alessio Manzoni blazed over seconds later.
Siena had their first half-decent attempt in the 28th minute but Lorenzo Del Prete's tantalising cross was headed out by Atalanta defender Gyorgy Garics and Maccarone just failed to get on the end of the resulting corner.
The visitors were struggling to make any real impression but could have taken the lead shortly before half-time, however Atalanta goalkeeper Ferdinando Coppola reacted swiftly to deal with Daniele Galloppa's long cross.
Galloppa was again in a good position 10 minutes after the break and this time tried his luck from the edge of the area but the solid Atalanta rearguard easily dealt with the ball.
Atalanta should have doubled their tally 10 minutes later but Thomas Manfredini's cross sailed out for a goal-kick as he tried to pick out Doni.
Manfredini was again guilty of wasting an opportunity seconds later as another misdirected cross failed to trouble Curci.
Siena thought they had equalised 10 minutes from time but Padoin cleared Mario Frick's effort off the line.
Any hopes the visitors had of snatching a point disappeared six minutes later as Maccarone was shown a second yellow card for dissent.
A moment of madness then saw the 28-year-old shove the referee for which he could be handed an even more severe punishment.
