In 1953, Mortensen founded his own industrial design studio, later renamed John Mortensen Associates. He worked on a wide range of projects, including household appliances, office equipment, furniture, toys and packaging.
Mortensen was a pioneer in packaging design, using bold graphics and bright colors to help products stand out on store shelves. He has worked with companies such as Coca-Cola, PepsiCo, Procter & Gamble and General Electric.
Mortensen also taught industrial design at Syracuse University for over 20 years and was a member of the American Institute of Graphic Designers. He has received numerous awards for his work, including the Gold Medal Award from the American Institute of Graphic Designers in 1973.