He started his career in law as a legal officer of the
National Bureau of Investigation, and thereafter as senior corporate attorney in the
Metropolitan Waterworks and Sewerage System.
Trial court judge Rosario was appointed senior assistant city prosecutor in
Quezon City in 1994, where he served for three years. Rosario became the presiding judge of Manila Metropolitan Trial Court Branch 19 in 1997, then later as presiding judge of the Makati Regional Trial Court Branch 66 until he was promoted to the Court of Appeals in September 2005.
Appellate court On September 12, 2005, President
Gloria Macapagal Arroyo appointed Rosario as an associate justice of the
Court of Appeals. Rosario was Chairman of The Court of Appeals' Ninth Division. He held the position for 15 years until his appointment to the
Supreme Court in 2020. == Associate justice of the Supreme Court ==