Mr. Hellmuth ran the St. Louis company from its Washington office, where he led design work on dozens of local projects including the National Air and Space Museum’s Steven F. Udvar-Hazy Center in Chantilly, Va., the mixed-use Constitution Square development in the District, and the renovation of Walter Reed National Military Medical Center in Bethesda, Md.
His other major projects included the 75-story Abu Dhabi National Oil Co. headquarters, the U.S. Embassy in Nairobi and the U.S. Embassy chancery and office annex in Moscow, but Mr. Hellmuth said he considered his Washington buildings to be his most significant work.
“The ability to help shape a city and have your buildings be a part of the fabric of that city — for me, that is really the most important stuff,” he told Building Design+Construction, an architecture magazine that produces an interview program on YouTube. “Building that body of work and really taking a city which I love and helping move it forward, that’s for me the most important thing.”
His other projects around the Washington region include the D.C. Consolidated Forensic Laboratory, the federal Center for Weather and Climate Prediction in College Park, Md., and a luxury shopping center in Chevy Chase, Md.
Mr. Hellmuth was an early leader in sustainable design, both at HOK and in the wider architecture world.
The D.C. forensic lab building, a 351,000-square-foot project, includes automated solar sunscreen covers and fritted glass louvers to control indirect light in open office areas. The weather center in College Park, for the National Oceanic and Atmospheric Administration, features a four-story waterfall for moving rainwater past an outdoor dining terrace and into bio-retention gardens.
During Mr. Hellmuth’s tenure, HOK completed hundreds of projects that were green-certified.
“It isn’t because we’re tree-huggers or doomsayers,” Mr. Hellmuth told Washington Business Journal in 2008. “It’s because sustainable design is now an essential part of architecture.”
“As architects,” he added, “we have a moral obligation to fix this before it has catastrophic consequences.”
William Koerner Hellmuth was born in Cleveland on June 3, 1953. His father was a vice president at Gascon Paper Co., and his mother was a homemaker.
He was 10 when his father died, and his paternal uncle, George Hellmuth, was one of HOK’s founders. Yet his father and uncle were estranged, so Bill was only vaguely aware of the Hellmuth side of the family, let alone its connection to architecture.
Mr. Hellmuth studied architecture at the University of Virginia, graduating in 1975. Two years later, he received a master’s degree in architecture from Princeton, where he was taught by Michael Graves, who helped define the postmodern movement in the field.
After he finished his schooling, “HOK wasn’t even on my radar,” Mr. Hellmuth told a journal published by the American Institute of Architects.
His first job was designing office towers at Skidmore, Owings & Merrill in New York City. After 15 years there, he got restless and a friend suggested he contact Gyo Obata — the O in HOK. Obata hired him, and Mr. Hellmuth finally got to know the other side of his family, including George Hellmuth.
“I always tell people I’m at HOK by accident,” he said in the book “Designing a World-Class Architecture Firm,” a history of HOK. “It’s a wonderful accident, but it was an accident.”
After working in St. Louis for two years, Mr. Hellmuth became the head of HOK’s Washington office in 1993. He became president of HOK in 2005 and chief executive in 2016. Last November, the firm announced he was on medical leave.
Mr. Hellmuth married Nancy LeSage in 1982. In addition to his wife, survivors include two children, Will Hellmuth of San Francisco and Grayson Rueckert of New York; two brothers; and two grandchildren.
In the days following his death, Mr. Hellmuth was being remembered in the architecture industry for his hands-on approach to projects even while running the company. He also continued to mentor up-and-coming architects.
Asked once what advice he would give to architecture students, Mr. Hellmuth said to be helpful wherever they land after school.
“You’re not going to be Frank Lloyd Wright on the first day,” he said. “Even Frank Lloyd Wright wasn’t Frank Lloyd Wright on the first day.”