He started working as a trial attorney at the Office of the Solicitor General (OSG) in 1985 and was awarded the Most Outstanding Solicitor three years later.[5]
In August 2002, he was promoted to assistant solicitor general. He continued to serve in the OSG until October 2005.[5]
After 20 years with the OSG, he was then appointed as an associate justice of the Sandiganbayan on October 15, 2005, later as chairperson of its seventh division and co-chairperson of the Committee on Rules.
On August 14, 2017, Gesmundo became the 178th Supreme Court magistrate, assuming the post vacated by Justice Jose C. Mendoza who reached mandatory retirement age.[6]
As chief justice, he will mandatorily retire on November 6, 2026.