Along with Gyo Obata and the late George E Kassabaum, Hellmuth started HOK in St Louis in 1955 with 26 employees. The company now employs more than 1600 architects, engineers and administrative staff in 24 offices worldwide.
Hellmuth began his career as a city architect designing police stations and bus shelters in St Louis. During the 1940s, he moved to Detroit where he worked for an architectural engineering firm that specialised in designing industrial buildings to support the US war effort.
HOK began as a small practice specialising in school and college buildings. Hellmuth was one of the driving forces behind its expansion into other services and disciplines.
HOK director Stephan Reinke said: "George was primary in our development as an international practice. He was the guy who invented the team approach to architecture.
"He was the development leg of the three-man stool. Kassabaum was the production expert and Obata was the designer. But Hellmuth was able to unlock the development potential of a project for clients. He was very good at understanding exactly what clients wanted."