Born in Turin, Piedmont, Italy, Pinelli began his career as a civil lawyer but spent his free time working in the theatre as a playwright. Descended from a long line of Italian patriots, his great-uncle General Ferdinando Pinelli quashed the bandit revolt in Calabria following Italian unification.

He first met Fellini in a Rome kiosk in 1947 while they were reading opposite pages of the same newspaper. "Meeting each other," explained Pinelli, "was a creative lightning bolt. We spoke the same language from the start... We were fantasizing about a screenplay that would be the exact opposite of what was fashionable then: the story of a very shy and modest office worker who discovered he can fly; so he flaps his arms and escapes out the window. It certainly wasn't Italian neorealism. But the idea never went anywhere either." The anecdote about flying presages their 1963 film, 8½, which begins with the protagonist, a prominent film director, who dreams of escape by flying out of his car caught in a traffic jam.

Pinelli died on March 7, 2009 in Rome. He was married to the French-born actress Madeleine LeBeau, who had small roles in 8½ and Casablanca (1942).