The eldest of three sons, Samelson was born on 3 March 1916, in Strassburg, Germany (now Strasbourg, France).
His family helped him leave Nazi Germany in 1936 for Zurich, Switzerland, where he studied with the geometer Heinz Hopf and received his doctorate in 1940 from the Swiss Federal Institute of Technology.
In 1941, he accepted a position at the Institute for Advanced Study in Princeton and immigrated to the United States; he arrived by ship six months before the United States entered World War II and acquired U.S. citizenship several years later.
Though he became emeritus in 1986, he remained professionally active throughout his retirement, publishing articles on both contemporary and historical mathematical topics.
He was active in the Palo Alto Friends Meeting (Quakers) during his retirement, serving as treasurer for several years.