Born in Cagliari, Sardinia, Mancosu started his career with hometown club Cagliari Calcio and made his first team debut in the last round of 2006–07 season, also scoring a goal in his first top-flight appearance, and then made 10 first team appearances in the following 2007–08 season.
He joined Serie B side Rimini Calcio on loan for the 2008–09 season in order to gain more first team experience.
[4] On 13 August 2010, it was confirmed that Mancosu was sold to Lega Pro Prima Divisione side U.S. Siracusa, on a co-ownership deal.
The Sicilian club, recently underwent a second consecutive promotion from the Serie D in 2008 to the Lega Pro Prima Divisione this season.
In January 2019, he was named the team's new captain, as his predecessor, Franco Lepore, had recently been sold by the club.