Hultberg primarily made Abstract expressionist paintings that were minimalist and dark, and also made Surrealist invented landscapes with “linear perspectives and angular shapes.” Hultberg was described as an Abstract realism, who combines “abstract” and “concrete” with attention to detail, bold color use, and strong design.
In 1952 he was introduced at the Museum of Modern Art in a show of new artists. He lived for one year in Paris between 1954 and 1955 and gained a reputation there for his work. In 1955 he won the Corcoran Biennial first prize in Washington.
His paintings were influenced by his time spent at Monhegan Island, and his career thrived after he moved to Portland. His work was shown in many galleries, including the Anita Shapolsky Gallery in New York City and the Albright Art Gallery in Buffalo, New York, he gave lectures, and in 1985 he had an exhibition at the Portland Museum of Art.
A play that he wrote was produced the University of Maine theatre department. He published the book Sole Witness, Vagabondage, a Paris Odyssey (1953–1955), his poetry and other books. He taught art in Hawaii and the West Coast.
He was teaching at the Art Students League and was a full-time resident in New York by 1990. He taught until the week of his death at the Art Students League. His work was part of a group show at Aucocisco in Portland in February 2005, at which time he was living in New York City.