He was the son of William and Charlotte née Pettigrew. He attended Washington & Jefferson College and obtained his Bachelor of Arts in 1889 from Johns Hopkins University and his PhD in Medicine in 1892 from the Medical School at the Columbia University College of Physicians and Surgeons. He did his specialization in Paris at the Institut Pasteur. In 1896 he married Mabel Houston with whom he had two children.
He was a physician at the Johns Hopkins Hospital in Baltimore starting in 1895; he studied malaria and yellow fever. In 1900 he reported for duty as the assistant surgeon at Columbia Barracks (Quemados) for the United States Army.
After a few months in Quemados, Lazear, together with Walter Reed (1851-1902), James Carroll (1854-1907) and Aristides Agramonte (1869-1931), participated in a commission studying the transmission of yellow fever, the Yellow Fever Board. During his research at Camp Colombia, he discovered that mosquitos transmitted this disease. A portion of his study, though, had been conducted on himself: without telling his colleagues, he had allowed himself to be bitten by yellow fever-infected mosquitoes and died of the disease at age 34. A dormitory at Johns Hopkins University was named after him in honor of his sacrifice, as was the chemistry building at Washington & Jefferson College, Lazear's alma mater.