His military service began shortly before the Armistice and he was therefore never sent overseas.
After leaving the army, Gummo, who in an interview said he never liked being on stage, went into the dressmaking business. Later he joined with his brother Zeppo Marx and operated a theatrical agency. After that collaboration ended, Gummo represented his brother Groucho Marx and worked on the television show The Life of Riley, which he helped develop. He also represented other on-screen talent and a number of writers.
Gummo was well respected as a businessman. He rarely had contracts with those he represented, his philosophy being that, if they liked his work, they would continue to use him, and if not, they would seek representation elsewhere. Unlike his brothers, his social life involved primarily business people.
Gummo was given his nickname because he had a tendency to be sneaky backstage, and creep up on others without them knowing (like a gumshoe). Another explanation cited by biographers and family members is that Milton, being the sickliest of the brothers, often wore rubber overshoes, also called "gumshoes," to protect himself from taking sick in inclement weather.
April 21, 1977, in Palm Springs, California. His death was not reported to Groucho, who by that time had become so ill and weak that it was thought the news would be of further detriment to his health. Groucho died four months later.
Gummo is entombed in the Freedom Mausoleum at Forest Lawn Memorial Park, Glendale in Los Angeles, California.
In the Jasper Fforde novel The Fourth Bear, there is a brief reference to GummoWorld, an amusement park dedicated to Gummo Marx.
In the Woody Allen film Stardust Memories, a woman at a film festival is referred to as having written the definitive filmography of Gummo Marx. Another enthusiast then observes, "Interestingly, he's the one Marx brother that never made any movies."
The 1997 Harmony Korine film Gummo was named after Gummo Marx, and there is one reference to his comedic style in a scene of the film.
Thomas Pynchon's 2009 novel Inherent Vice includes an L.A. street named "Gummo Marx Way" (pp 283, 284).