After the war he developed into an expressionist painter, influenced by Van Gogh, Matisse and Picasso, among others. He was involved with the Experimentals in 1947 and with the CoBrA group from 1948. He was good friends with members of the Cobra movement, including Karel Appel and Corneille, but members of CoBrA thought he was too young to officially join the art movement. Sierhuis worked mainly abstractly, but was more figurative than Appel and Corneille.
"Without figuration I would never have known exactly what to talk about, a grasp. For me, life is that coat. I paint from there. Never from a formalism or a theory."
Later was hij medeoprichter in 1962 van de Amsterdamse kunstenaarsgroep Groep Scorpio; "Angry Young men die het tentoonstellen anders wilden aanpakken. In die periode begon hij ook naam te maken; Willem Sandberg van het Amsterdamse Stedelijk Museum begon zijn werk te exposeren en ook aan te kopen wat resulteerde in zijn eerste grote overzichtstentoonstelling aldaar in 1971. Daarvoor exposeert hij al in Parijs, Mexico, Brazilië en Kopenhagen.
After 1984 he returns to romantic expressionism, in large formats he is inspired by flamenco, which fascinates him mainly because of the feeling that this dance conveys. These dances have everything from his painting: color, emotion, movement and rhythm. He spent many summers in Spain where a passion for flamenco arose and he also started working in three dimensions. He has built up a broad oeuvre of paintings, gouaches, sculptures, etchings, lithographs and graphics.
Jan Sierhuis taught at Atelier '63 in Haarlem and later after 1983 at the Rietveld Academy and the Rijksacademie in Amsterdam. He received several prizes, including an honorable mention at the Prix de Rome. In 1990 he had an exhibition at Delaive. In 1998, the Stedelijk Museum in Amsterdam put together a major exhibition in honor of De Sierhuis' seventieth birthday. In 2002 he became an Officer in the Order of Orange-Nassau.