He was noted particularly for his work on enzymes, using concepts drawn from organic chemistry to understand their mechanisms.

Jencks conducted his first postdoctoral research for two years with Fritz Lipmann at Harvard Medical School.

He proposed that enzymes use ground state destabilization, termed the Circe Effect, to increase the reactivity of their bound substrates.

He was memorialized at the 20th Enzyme Mechanisms Meeting in St. Pete Beach, Florida, several days after his death.

After his first year at medical school, he did research with George Wald at the Marine Biological Laboratory at Woods Hole on lobster shell pigments.