Born in New York City, Wald was educated at New York University and at Columbia where he obtained his PhD in 1932. After spending the period 1932–34 in Europe, where he worked under Otto Warburg in Berlin and Paul Karrer in Zurich, he returned to America where he took up an appointment at Harvard. Wald remained at Harvard for the whole of his career, becoming professor of biology in 1948 and emeritus professor in 1977.

Wald did fundamental work on the chemistry of vision. In 1933 he discovered that vitamin A is present in the retina of the eye, and thereafter tried to find the relationship between this vitamin and the visual pigment rhodopsin. The first clue came from the constitution of rhodopsin. It was found to consist of two parts: a colorless protein, opsin, and a yellow carotenoid, retinal, which is the aldehyde of vitamin A. Wald was now in a position to work out the main outlines of the story.

Rhodopsin is light sensitive and splits into its two parts when illuminated, with the retinal being reduced further to vitamin A by the enzyme alcohol dehydrogenase. In the dark the procedure is reversed. What was further needed was some indication of how the splitting of the rhodopsin molecule could somehow generate electrical activity in the optic nerve and visual cortex. Part of the answer came from Haldan Hartline and Ragnar Granit who shared the 1967 Nobel Prize for physiology or medicine with Wald.

Wald speculated that since retinal is a carotenoid pigment, and such pigments are also found in plants, then it is possible that the phototropic responses of plants may rely on a similar mechanism.

Wald later became widely known for his opposition to the Vietnam War.