Born Meshilem Meier Weisenfreund in Lemberg, Austria (now Lviv, Ukraine), his family emigrated to the United States in 1902. Both of Muni's parents were actors with the Yiddish theater and it was only natural that he would join them on stage. He made his stage debut at the age of twelve, and was quickly recognized by Maurice Schwartz, who signed him up with his Yiddish Art Theater. In a stage quirk, Muni played an eighty-year-old man as his first role. It was an appropriate beginning for an actor who loved wearing elaborate costumes and assuming accents in his film career.
Muni was twenty-nine when he began acting on Broadway. His first role, that of an elderly Jewish man in the play We Americans by Sam Harris, was also the first time that he ever acted in English. He was signed by Fox three years later, in 1929, and received an Oscar nomination for his first film The Valiant, but he was unhappy with the roles and decided to return to Broadway.
In 1932 Muni returned to Hollywood to star in Scarface and I Am a Fugitive from a Chain Gang (1933), for which he received a second Best Actor nomination. Warner Brothers recognized his considerable talent and signed a long-term contract with him. He went on to receive a Best Actor nomination for The Life of Emile Zola (1937) and to win the Academy Award for The Story of Louis Pasteur (1939).
In the ensuing years until his retirement in 1959, he spent his time acting on film and stage. He was widely recognized as a talented if eccentric individual. He would go into a rage whenever anyone wore red, but at the same time he could often be found between sessions relaxing with his violin. Over the years, he also because increasingly dependent on his wife Bella, who terrified directors by forcing them to redo scenes that did not meet her satisfaction.
After several failed projects, Muni was nominated for a Tony Award in 1955 for the role of Henry Drummond in Inherit the Wind. He was becoming increasingly infirm, however, and by 1959, soon after receiving his final Academy Award nomination for The Last Angry Man (1959) he retired from acting. He died in Montecito, California.