Bemelmans was born to the Belgian painter Lambert Bemelmans and German Frances Fischer in Meran, Austria (now Merano, Italy). His father owned a hotel. He grew up in Gmunden on the Traunsee in Upper Austria. His first language was French and his second German.
In 1904, his father left the family for Ludwig's governess. His mother Frances took Ludwig and his brother to her native city of Regensburg, Germany. Bemelmans had difficulty in school as he hated the German style of discipline. He was apprenticed to his uncle Hans Bemelmans at a hotel in Austria, but he was said to shoot and seriously wound a waiter. He had to choose between being institutionalized or emigrating to the United States.
He spent the next several years working at hotels and restaurants in the US. In 1917, he joined the U.S. Army but was not ordered to Europe because of his German origin. He did become an officer, and was promoted to second lieutenant. In 1918, he became naturalized as an American citizen.
In the 1920s, Bemelemans tried to become an artist and painter while working at hotels, but had substantial difficulties. His cartoon series The Thrilling Adventures of the Count Bric a Brac was dropped from the New York World after six months. He associated with Ervine Metzl, a commercial artist and illustrator who is variously described as Bemelmans' friend, "agent", and "ghost artist".
In the early 1930s Bemelmans met May Massee, the children's book editor at Viking Press, who became a sort of partner. He began to publish children's books, beginning with Hansi in 1934. He published the first Madeline book in 1939; after being rejected by Viking, it was published by Simon and Schuster.
In 1953, he fell in love with a small bistro in Paris, "La Colombe" in the Ile de la Cité, and bought it. He painted murals in the bistro and owned the place for two years before selling it to Michel Valette, who converted it into a notable cabaret.
Bemelmans also wrote a number of adult books, including travel and humorous works, as well as movie scripts. The latter included Yolanda and the Thief. While spending time in Hollywood, he became a close friend of the interior decorator Elsie de Wolfe, Lady Mendl.
Bemelmans' Central Park, a mural on the walls of the Carlyle Hotel's Bemelmans Bar in New York City, is his only artwork on display to the public. He painted the children's dining room on Aristotle Onassis yacht Christina (now the Christina O), for the young daughter of the magnate, Christina Onassis.
Bemelmans is buried in Arlington National Cemetery (Section 43, Grave 2618).