After graduating medical school Heiser, with interest in leprosy, became the Philippine director of Health.

After joining the Public Health Service, he soon was screening immigrants for infectious diseases at Ellis Island and in Italy.

He implemented public health programs to combat smallpox, plague, cholera, malaria, beriberi, leprosy, and other afflictions.

[3] He was an eyewitness source to historian David McCullough for his 1968 book, "The Johnstown Flood", built the public-health system for the American colonial government in the Philippines between 1903 and 1915 and later worked for the Rockefeller Foundation.

On October 27, 1902, Heiser became the Philippine Director of Health and took over authority for establishing a leprosarium, called the Culion leper Colony.

Heiser served under Governor-General of the Philippines Leonard Wood (October 14, 1927, to August 7, 1927) for one year before being replaced by Herbert Windsor Wade as Medical Director (1922 to 1959) and the colony was finally reinstated into the population in 1998.

[7] Heiser married a wealthy widow, Marion Peterson Phinny, and they divided their time between New York and Connecticut until her death in 1965.