After being ordained to the Reformed Church ministry by the Pella, Iowa Classis in 1890, he was a missionary at Busrah, Bahrein, and at other locations in Arabia from 1891 to 1905. He was a member of the Arabian Mission (1890–1913). Zwemer served in Egypt from 1913–1929. He also traveled widely in Asia Minor, and he was elected a fellow of the Royal Geographical Society of London.
In 1929 he was appointed Professor of Missions and Professor of the History of Religion at the Princeton Theological Seminary where he taught until 1937. He had married Amy Elizabeth Wilkes on May 18, 1896. He was famously turned down by the American Missionary Society which resulted in him going overseas alone. He founded and edited the publication The Moslem World for 35 years. He was influential in mobilizing many Christians to go into missionary work in Islamic Countries.
Zwemer retired from active work on the faculty of Princeton College Seminary at the age of seventy, but continued to write and publish books and articles as well as doing a great deal of public speaking. Zwemer died in New York City at the age of eighty-four.
According to Ruth A. Tucker, Samuel Zwemer’s converts were “probably less than a dozen during his nearly forty years of service” and his “greatest contribution to missions was that of stirring Christians to the need for evangelism among Muslims.” — Wikipedia