Monteiro started his medical career in
Tan Tock Seng Hospital (TTSH) in 1929. During the
Japanese Occupation, Monteiro was Director of Middleton Hospital for Infectious Diseases, which was TTSH's infectious diseases wing and a predecessor of
National Centre for Infectious Diseases in Singapore. He discovered that the available quantity of
diphtheria antitoxin was depleting. He exposed live goats to diphtheria to create more antitoxin. After completing his post-graduate studies on a Queen's Scholarship in 1949, he was elected head of Faculty of Medicine of the then University of Malaya, Singapore in 1956–1960. In 1958, he started to use Sabin vaccine on a mass scale to protect young children and adults from
polio to wipe-out beri-beri with Vitamin B-1 and eliminated
diphtheria in children. By 1977, diphtheria and polio had become things of the past for Singaporeans. Upon his retirement in 1965, he was appointed the
Emeritus Professor and
Pro-Chancellor of
National University of Singapore. Monteiro taught many medical students from Singapore and Malaysia. Among his students were
Mahathir Mohamad (former Prime Minister of Malaysia) and
Siti Hasmah Mohamad Ali. He played a major role in establishing high standard of medical practice in Singapore. Monteiro was appointed the first Ambassador of Singapore to Cambodia in 1966–1968 and to United States and Brazil in 1969–1976. Monteiro came back home in 1977 and continued his private practice as a physician. ==Family and death==