Sumner, a wealthy cotton manufacturer's son from Canton, Massachusetts, was educated at Harvard, where he obtained his PhD in 1914. In the same year he took up an appointment at the Cornell Medical School where, in 1929, he became professor of biochemistry.

Despite having lost an arm in a shooting accident at 17, Sumner persisted in his desire to become an experimental chemist. In 1917 he began his attempt to isolate a pure enzyme. He chose for his attempt urease, which catalyzes the breakdown of urea into ammonia and carbon dioxide and is found in large quantities in the jack bean. After much effort he found, in 1926, that if he dissolved urease in 30% acetone and then chilled it, crystals formed. The crystal had high urease activity. Moreover Sumner's crystals were clearly protein and however hard he tried to separate the protein from them he always failed. He was therefore forced to conclude that urease, an enzyme, was a protein. However, this ran against the authority of Richard Willstätter who had earlier isolated enzymes in which no protein was detectable. In fact, protein was in Willstätter's samples, but in such small quantities as to be undetected by his techniques.

Consequently little attention was paid to Sumner's announcement and it was only when John Northrop succeeded in crystallizing further protein enzymes in the early 1930s that his work was properly acknowledged. In 1946 for “his discovery that enzymes can be crystallized” he was awarded the Nobel Prize for chemistry jointly with Northrop and Wendell Stanley.