After working in private practice and in the House of Representatives of Puerto Rico, Martínez in 1995 was appointed to the Puerto Rico Court of Appeals by Governor Pedro Rosselló. After spending 14 years in that Court, Governor Luis Fortuño elevated him to Associate Justice of the Supreme Court to fill the vacancy left by Justice Baltasar Corrada del Río. That seat had been vacant for over three years.
He was sworn in on March 10, 2009. The Constitution of Puerto Rico mandates that Supreme Court Justices must retire when they reach the age of seventy. Therefore, Justice Martínez term will expire in 2029.