He began his career with fellow artists Peyo and Morris, who would also go on to become renowned. He took over the Spirou strip in 1946, and largely reinvented it, creating longer, more elaborate storylines and a plethora of burlesque characters.
Most notable is the Marsupilami, which was inspired by seeing busy tram conductors on his way into work with his colleagues. This fictional animal has become part of (French? Franco-Belgian? European) popular culture, and has spawned cartoons, a comic book series of its own and merchandise. The cartoons have broadened its appeal to English-speaking countries.
In 1957 he created anti-hero Gaston as a joke. The weekly strip, detailing the mishaps and madcap ideas and inventions of a terminally idle office boy took off, and became Franquin's best-known creation. He continued work on the strip until his death.
His Ideés Noires (lit. "Dark Thoughts") strips concentrated on these themes, and showed a darker, alarmed side of his nature. In one strip , a pair of flies are seen wandering a strange looking landscape, discussing the mistakes of their predecessors. In the final box, we see the landscape is a city made from human skulls, and one fly says "don't be too hard on them, they did leave us such splendid cities".
See also: Franco-Belgian comics artists