A Gutai member from 1955 to 1971. A self-taught artist, he experimented with abstract painting early in his career and made experimental sculptures using various natural and industrial materials such as wood, rocks, water, plastics, and nails. Some of the site-specific works he created for open-air Gutai exhibitions are today recognized as installations. Inspired by tarashikomi, a traditional Japanese painting technique, Montonaga poured different colored paints on canvas without blending them. When living in New York from 1966 to 1967, he developed a technique using airbrush and acrylic paint that prepared the way for his later hard-edge paintings, with their bright, saturated colors and humorous titles. His 2011 reconstruction of Work (Water) for the Guggenheim Museum’s 2013 exhibition Gutai: Splendid Playground resurrected a key piece in his oeuvre: a vividly colored, site-specific work that he originally created in 1956. Motonaga was also known as a designer of chairs and an illustrator of children's books.
A retrospective exhibition titled "Between Action and the Unknown: The Art of Kazuo Shiraga and Sadamasa Motonaga" was held in Dallas Museum of Art in 2015.