Forest in 1932 received his doctorate at Columbia University for a PhD in zoology. This was followed by research stays at Otto Warburg in Berlin, with Paul Karrer in Zurich and at Otto Fritz Meyerhof in Heidelberg. As a Jew, he had to leave Germany in 1933 because of the rise of the Nazis. He first came to the University of Chicago, then in 1934 at Harvard University. He later became a professor. In 1950 he was elected a member of the National Academy of Sciences.
Forest received the Albert Lasker Award in 1953 for Basic Medical Research in 1967 and together with Ragnar Granit and Haldane Keffer Hartline received the Nobel Prize for Medicine for the study of physiological and chemical visual processes in the eye.
He succeeded, among other things deciphering the function of vitamin A ( discovery of vitamin A1 = Retinol and Vitamin A2 = dehydroretinol in the retina ) and the visual purple or rhodopsin in the rods of the retina.