Upon arriving back in the United States in 1955, Diffrient joined Dreyfuss Associates in their Pasadena offices, and spent the next few decades revolutionizing American industrial design through his development of interiors and corporate identity for American Airlines planes, the landmark Princess telephone, the Polaroid SX-70 camera, and tractor seats for John Deere. In 1980 after 25 years with Henry Dreyfuss Associates, Diffrient left to start his own independent practice, establishing a design studio in Ridgefield, Connecticut which he shared with his wife, tapestry artist Helena Hernmarck.