Windaus studied medicine at the university in his native city of Berlin and at Freiburg University, where he changed to chemistry under the influence of Emil Fischer. After holding chairs in Freiburg and Innsbruck he became, in 1915, professor of chemistry at Göttingen, where he remained until his retirement in 1944.

In 1901 Windaus began his study of the steroid cholesterol, a compound of considerable biological significance. Over the years he threw considerable light on its structure and in 1928 was awarded the Nobel Prize for chemistry for this work and for showing the connection between steroids and vitamins.

It was known that cod-liver oil prevents rickets because it contains vitamin D. It was also known that sunlight possesses antirachitic properties and, further, that mere exposure of certain foods to sunlight could make them active in preventing rickets. Clearly something in the food is converted photochemically into vitamin D but nobody knew what.

As vitamin D is fat soluble, the precursor of vitamin D (the provitamin) was not surprisingly found to be a steroid. In 1926 Windaus succeeded in showing that the provitamin is present as an impurity of cholesterol, ergosterol, which is converted into vitamin D by the action of sunlight.