The mixed jury retired for 58 minutes and returned a five-to-two verdict on both counts.
He was, however, found not guilty of rape.
High Court Judge Vivene Harris then ordered the former Special Sergeant remanded until March 13, when he is to be sentenced.
The court heard that in 2012, the accused met the complainant at a party in the Corporate Area.
He offered her a ride, and during the journey, he reportedly made sexual advances which were spurned.
The accused allegedly held her and sexually assaulted her.
A report was made to the police, and following an investigation, he was arrested and charged with buggery and rape.
He was on bail prior to the guilty verdict.