Ronis was born in Paris; his father was a Jewish refugee from Odessa, and his mother was a refugee from Lithuania, both escaped from the pogroms. His father opened a photography studio in Montmartre, and his mother gave piano lessons. The boy's early interest was music and he hoped to become a composer. Returning from compulsory military service in 1932, his violin studies were put on hold because his father's cancer required Ronis to take over the family portrait business; Ronis' passion for music has been observed in his photographs. His father died in 1936, whereupon the business collapsed and Ronis went freelance, his first photographs being published in Regards. In 1937 he met David Szymin and Robert Capa, and did his first work for Plaisir de France; in 1938–39 he reported on a strike at Citroën and traveled in the Balkans. With Cartier-Bresson, Ronis belonged to Association des Écrivains et Artistes Révolutionnaires, and remained a man of the left.

The work of photographers, Alfred Stieglitz and Ansel Adams inspired Ronis to begin exploring photography. After his father's death, in 1936, Ronis closed the studio and joined the photo agency Rapho, with Brassaï, Robert Doisneau and Ergy Landau.

Ronis became the first French photographer to work for Life.[citation needed] In 1953, Edward Steichen included Ronis, Henri Cartier-Bresson, Robert Doisneau, Izis, and Brassaï in an exhibition at the Museum of Modern Art titled Five French Photographers. In 1955, Ronis was included in the Family of Man exhibition. The Venice Biennale awarded him its Gold Medal in 1957. Ronis began teaching in the 1950s, and taught at the School of Fine Arts in Avignon, Aix-en-Provence and Saint Charles, Marseilles. In 1979 he was awarded the Grand Prix des Arts et Lettres for Photography by the Minister for Culture. Ronis won the Prix Nadar in 1981 for his photobook, Sur le fil du hasard.

Ronis' wife, the Communist militant painter Marie-Anne Lansiaux (1910–91), was the subject of his well-known 1936 photograph, Nu provençal (Provençal nude). The photograph, taken in a house that he and Anne Marie had just bought in Gordes, showed Marie-Anne washing at a basin with a water pitcher on the floor and an open window through which the viewer can see a garden, this is noted for its ability to convey an easy feeling of Provençal life. The photograph was a "huge success"; Ronis would comment, "The destiny of this image, published constantly around the world, still astonishes me." Ronis lived in Provence from the 1960s to the 1980s.

Late in her life, Ronis photographed Marie-Anne suffering from Alzheimer's disease, sitting alone in a park surrounded by autumn trees.

Ronis' nudes and fashion work (for Vogue and Le Jardin des modes) show his appreciation for natural beauty; meanwhile, he remained a principled news photographer, resigning from Rapho for a twenty-five year period when he objected to the hostile captioning by the New York Times to his photograph of a strike.

Despite stiff competition from Robert Doisneau and others, the Oxford Companion to the Photograph terms Ronis "the photographer of Paris par excellence".

Ronis continued to live and work in Paris, although he stopped photography in 2001, since he required a cane to walk and could not move around with his camera. He also worked on books for the Taschen publishing company.

In 2005-2006 the City of Paris presented "Willy Ronis in Paris", a big retrospective show of his work, that had a huge success with over than 500.000 visitors.

Ronis died at age 99, on September 12, 2009.