Loeb, son of a Jewish family from the German Eifel region, was born in 1869 in Mayen, Kingdom of Prussia.

Indeed, over the succeeding couple of years, he spent only brief periods at several universities, in Berlin, Freiburg, and Basel, unable to focus his interests.

[1] His brother had a teaching position at the University of Chicago, and Leo moved to that city from Europe after completing his medical degree.

[3] Loeb was next asked to join the faculty at McGill University in Montreal; however, that posting lasted only one year because of his inability to acclimatize himself to the harsh Canadian winter.

He moved to the University of Pennsylvania in Philadelphia in 1903, beginning a series of important experiments on the influence on cancer growth by reproductive hormones.

Having long deferred a family life to concentrate on his research, he finally married Dr. Georgiana Sands (another physician) in 1922, at the age of 53.

In 1924, Loeb was given the chairmanship of pathology at WUSM; thereafter, he continued his work on tissue transplantation and cell culture, as well as research on endocrine disease.