Magnus had one daughter, Gretl Magnus, who became a translator in Berlin and who died in 1968. She was married to Walter Zander and together they had a son, the composer Michael Zander. Rudolf Magnus' son, Otto Magnus wrote his biography in 2002, entitled Rudolf Magnus, Physiologist and Pharmacologist: A Biography.
Magnus is most widely known for his work as a physiologist. His book Körperstellung ("Posture")., a study of functional neurology, is his best known work.
In 1901, while in Germany, Magnus discovered the diuretic effect of the excretions of the pituitary gland. From 1908, Rudolf Magnus worked in Liverpool on the physiology of posture and muscle tension. Although he was a pharmacologist, this research made him world famous. For his work, he was nominated for the Nobel Prize in 1927. His sudden death in 1927 made it impossible to award the prize to him. His most famous book, Körperstellung, was published in Berlin in 1924, and translated into English in 1987. In this book Magnus describes the reflexes involved in mammal posture. The Magnus & De Kleijn reflexes are named after Magnus and his colleague de Kleijn. The head and neck reflexes of mammals cause the body to follow automatically when the head moves. He also researched the reflexes of the intestines and phenomena such as motion sickness.
The pharmacological research of Rudolf Magnus was focussed on the effect of medication on the heart, blood vessels, lungs and the gastrointestinal tract. Thus he studied the effects of narcotics, as well as poison gasses on the lungs. He conducted the poison gas study during World War I (1914—1918) when he served as an army doctor in Germany.
It is said that Magnus was very fond of ice skating and would give his whole laboratory staff time off when the temperature was below freezing.
The Rudolf Magnus Intitute for Neuroscience still exists and is one of the research institutes of the University Medical Center Utrecht, where neuroscientific research is performed. The Anatomical Museum in Utrecht houses Rudolf Magnus' archive.