Upon his return to the Philippines, he joined the Bureau of Public Works where he initially worked as a draftsman in the agency from 1912 to 1917. He was later appointed as the supervising architect for the Bureau from 1917 to 1928. He spearheaded many government projects including the Philippine General Hospital Nurses Home, Psychopathic Building (National Mental Hospital) and the School for the Deaf and Blind. He also designed the Manila Central Post Office Building in Ermita, Manila. Tomás became known for his great contributions in the field of architecture.