His wife, Dale Van Dyke, said the cause was complications of bladder cancer.
When Armstrong died in 1971, he left behind 72 cartons packed with artifacts from his decades as probably the most celebrated figure in jazz. Inside the boxes were 650 reel-to-reel tape recordings of songs, ideas and conversations; at least 5,000 photographs; 86 scrapbooks; 240 acetate disks of live recordings that he made at home; five trumpets; and 14 mouthpieces.
Mr. Cogswell, a saxophonist whose master’s thesis was on four solos played by the pioneering free-jazz saxophonist Ornette Coleman, knew little about Armstrong when he answered a newspaper ad in 1991 for the archivist job. But after spending three years cataloging the archive at Queens College — Armstrong’s wife, Lucille, had bequeathed the house to the college — he had become a devoted Satchmo fan and expert.
And, consumed by the life and career of Armstrong, Mr. Cogswell rarely played the saxophone again.
“Before the job, there were two people in this marriage,” Ms. Van Dyke said in a phone interview. “When Louis came into Michael’s life, he came into my life, and all of a sudden there were three people in this marriage, and that was fine with me.”
But the archive was only the start of Mr. Cogswell’s 27-year association with Armstrong’s legacy. Over the next nine years, as the executive director of what would become the Louis Armstrong House Museum, Mr. Cogswell worked with a small staff on a $5 million renovation that preserved the house — lavishly decorated, on 107th Street in Corona, a working-class neighborhood — as if the Armstrongs were still living there and created a museum inside it.
In 2009, Mr. Cogswell reflected on his fascination with Armstrong. “I haven’t hit bottom yet,” he told the Queens College website. “Louis Armstrong is endlessly interesting. He was a trumpet player, vocalist, actor and author. But for all these accomplishments, what inspires me is Louis Armstrong, the person.
“He was a beautiful guy. He was humble. He was generous. He was a genius.”
Michael Bruce Cogswell was born on Sept. 30, 1953, in Buffalo, N.Y., and raised in Fairfax County, Va. His father, Charles, was a marketing consultant and a former brigadier general in the Marine Corps. His mother, Margaret (Hoyt) Cogswell, was a homemaker.
After three semesters at the University of Virginia, he dropped out in 1973 and played with bands in Charlottesville, Va., and Boston. Later returning to the university, he received a bachelor’s degree in musicology in 1983.
He earned a master’s in jazz studies at the University of North Texas, in Denton, and played locally with the Pinky Purinton big band. He worked in the university’s music library, where he organized the bandleader Stan Kenton’s collection, which included some 2,000 manuscripts.
Looking for his next step, he responded to an advertisement in an education publication for an archivist to handle the Armstrong collection. “He told me, ‘That’s my job,’” Ms. Van Dyke recalled.
Once he started working at Queens College, Mr. Cogswell earned a master’s in library science.
Loren Schoenberg, senior scholar and founding director of the National Jazz Museum in Harlem, said an email that Mr. Cogswell’s development of the archive and museum helped elevate Armstrong’s reputation beyond being a beloved entertainer, making many more people aware that he was the “prime architect of jazz.”
Mr. Cogswell used his knowledge of the Armstrong archive to write the book “Louis Armstrong: The Offstage Story of Satchmo” (2003).
One more project occupied Mr. Cogswell for more than a decade: building a $23 million education center across the street from the Armstrong house. It will house the archives, an exhibition gallery, a jazz club and a museum store. It broke ground in 2017, nearly a year before illness caused Mr. Cogswell to retire. (The coronavirus pandemic has halted construction.)
In addition to his wife, he is survived by two brothers, Frank and Charles.
Armstrong’s personal archive has been augmented over the years by donations of photographs, correspondence and recordings. But perhaps the most unusual donation came in 1997, when Mr. Cogswell received a call from a woman named Dorothea Vunk.
She offered a gift: the trumpet that Armstrong had received in 1934 from King George V. Several years later, Armstrong gave it to Ms. Vunk’s husband, Lyman, a young member of Charlie Barnet’s band.
“Michael said, ‘Thanks, where do you want to meet?’” David Ostwald, the former chairman of the Armstrong museum, said by phone. “She said, ‘I’ll meet you at the subway.’ Michael had all these official papers, but she had the trumpet in a paper bag and surreptitiously handed it to him, and she quickly disappeared.”