Panganiban was named as associate justice of the Supreme Court in 1995. Justice Panganiban was the chairperson of the Supreme Court Third Division and the House of Representatives Electoral Tribunal (HRET), as well as of seven SC committees involved mainly in judicial reforms. Described by a colleague (Justice Antonio T. Carpio) as ‘undoubtedly the most prolific writer of the Court, bar none’ he has during the last ten years penned more than 1,000 full-length decisions and ten books plus several thousand minute resolutions disposing of controversies.

Justice Panganiban was named Supreme Court Chief Justice on December 20, 2005 by President Gloria Macapagal-Arroyo after Hilario Davide, Jr. retired at age 70. Other nominees by the Judicial and Bar Council include Senior Associate Justice Reynato Puno and Associate Justice Leonardo Quisumbing. Puno is the most senior in terms of career in the Supreme Court while Panganiban is the most senior in terms of age.