Johannes Postma was born in 1935 in Zwagerbosch, the Netherlands, and raised by his grandparents. His elementary education was begun during Nazi occupation and interrupted when troops commandeered his schoolhouse for a barracks. At age thirteen, during the post-war hyperinflation, Postma had to leave school to help support his grandparents and himself. He continued his education through independent study and night courses, and although he never attended high school, Graceland College (now Graceland University, in Lamoni, Iowa) admitted him in 1958.

Using the trade of house painting learned from uncles, Postma worked his way through college by painting during summers, weekends, and holidays. After earning a bachelor’s degree from Graceland College in 1962 and a master’s degree from the University of Kansas in 1964, he joined the faculty at Western Michigan University. In 1966, he became a U.S. citizen and began doctoral studies at Michigan State University.

Wanting to combine archival research in the Netherlands with African history, and learning that little had been written about Dutch involvement in the transatlantic slave trade, Postma chose that subject for his doctoral dissertation. He spent the academic year 1967-68 at Dutch archives and studying maritime history at Leiden University. In 1969, he joined the history faculty at Mankato State College (now Minnesota State University at Mankato).

With a doctoral dissertation titled “The Dutch Participation in the African Slave Trade: Slaving on the Guinea Coast, 1675-1795,” Postma received his Ph.D. from Michigan State University in 1970. His treatise came on the heels of Philip Curtin’s seminal The Atlantic Slave Trade: A Census (University of Wisconsin Press, 1969), a synthesis of what was known about the transatlantic slave trade based on data then available in print. Because there had been no comprehensive study of the Dutch role before Postma’s, Curtin’s book lacked information on that aspect of the trade. The two men learned of each other’s work at an African Studies conference in Boston in the spring of 1970, where Postma presented his initial findings on Dutch participation in the African slave trade. Postma’s work, along with new work by a British scholar, enabled Curtin to modify his original conclusions at an international conference in Rochester, N.Y., in 1972 (Curtin, “Measuring the Atlantic Slave Trade,” in Stanley L. Engerman and Eugene D. Genovese, eds., Race and Slavery in the Western Hemisphere: Quantitative Studies [Princeton: Princeton University Press, 1975], pp. 104-28; “Measuring the Atlantic Slave Trade Once Again: A Comment,” The Journal of African History Vol. 17, No. 4 [1976], pp. 595-605).

During the 1972-73 academic year, Postma continued research in Dutch archives and traveled to Africa to visit slave trafficking sites in Ghana and Dahomey (now Benin). Having been introduced by Curtin to quantitative historical analysis, he studied the new approach at the University of Michigan and then organized sessions on the Atlantic slave trade for the 1974 International Congress on Economic History held in Copenhagen, where he offered a quantitative assessment on the Dutch slave trade. Postma continued archival work in the Netherlands during subsequent summers and taught a seminar at Leiden University during the 1986-87 academic year.

In 1990 Cambridge University Press published The Dutch in the Atlantic Slave Trade, 1600-1815, the results of Postma’s two decades of research and study. Shortly thereafter, Postma donated his research data to the Database of Slaving Voyages being collected by Harvard University’s W. E. B. Du Bois Institute, Voyages: The Trans-Atlantic Slave Trade Database. Originally published on CD-ROMs, the database was introduced to the public at the 1998 Transatlantic Slaving and African Diaspora Conference at Williamsburg, Virginia, where Postma chaired a session on Shipboard Experience. The collection was subsequently expanded into the open-access web site developed by David Eltis and other researchers at Emory University Slave Voyages and will continue to be updated as new data becomes available.

After publication of his magnum opus on the slave trade, Postma turned his attention to the Netherland’s commerce in general in the Atlantic, focusing on Surinam, a former Dutch colony on the north coast of South American. In 1995 he received a fellowship at the Netherlands Institute of Advanced Study (NIAS) to pursue that work, and he produced several papers on the subject before retiring from teaching in 2001. In retirement, Postma coedited with former student Victor Enthoven Riches from Atlantic Commerce: Dutch Transatlantic Trade and Shipping, 1585-1817 (Leiden, Brill, 2003), a collection of essays by nine scholars. Postma contributed three chapters, including the results of his Surinam work, “Suriname and Its Atlantic Connections, 1667-1795.” He also wrote two textbooks for Greenwood Press’s Guides to Historic Events, 1500-1900: The Atlantic Slave Trade (2003) and Slave Revolts (2008).

Postma also published a personal memoir, At Home in a Barn: From Dairy Barn to Cozy Home (Lulu Publishing, 2015), about his conversion of a Minnesota dairy barn into a home. He had purchased the barn and commenced renovation in 1976, after his divorce from fellow Graceland College student Sharon King (1962-74), the mother of his two sons, Mark and Peter. Most of the work on the barn was completed by 1980 when he married Laurel Menne, whose surname he adopted as a middle name during their marriage (1980-92). She designed the maps, diagrams, and jacket cover for The Dutch in the Atlantic Slave Trade.

In 1993 Postma married Joelle Million, a fellow history scholar and biographer of nineteenth-century women’s rights leader Lucy Stone (Woman’s Voice, Woman’s Place: Lucy Stone and the Birth of the Woman’s Rights Movement, Praeger, 2003). They reside in Western Massachusetts and continue to pursue their writing projects together.