Born in Berlin (Germany), he studied medicine in Freiburg, where he graduated in 1897. He then moved to Berlin, where he received his doctorate the same year. Michaelis worked as assistant to Paul Ehrlich (1898–1899), Moritz Litten (1899–1902) and Ernst Viktor von Leyden (1902–1906). In 1906 he started as director of the bacteriology lab in Berlins Charité hospital, becoming Professor extraordinary at Berlin University in 1908.
In 1914 he published a paper suggesting that Emil Abderhalden's notorious pregnancy tests could not be reproduced, a paper which fatally compromised Michaelis' position as an academic in Germany (L. Michaelis, L von Lagermark, Deutsche Med. Wochenschr. 1914, 7, 316-319). In 1922 he moved to the Medical School of the University of Nagoya (Japan) as Professor of biochemistry, in 1926 to Johns Hopkins University in Baltimore, Maryland as resident lecturer in medical research and in 1929 to the Rockefeller Institute of Medical Research in New York City, where he retired in 1941.
Besides his role in the formulation of the Michaelis-Menten equation (1913) he discovered Janus green as a supravital stain for mitochondria and the Michaelis-Gutmann body in urinary tract infections (1902) and found that thioglycolic acid could dissolve keratin, making him the father of the permanent wave.
He died in New York City.