"I couldn't have had a more brilliant TV partner," tweeted Mario Cantone, who played Garson's husband in Sex and the City. "I'm devastated and just overwhelmed with sadness."
He added: "You were a gift from the gods."
"Willie Garson was in life, as on screen, a devoted friend and a bright light for everyone in his universe. He created one of the most beloved characters from the HBO pantheon and was a member of our family for nearly 25 years," an HBO Facebook statement said. "We are deeply saddened to learn of his passing and extend our sincere condolences to his family and loved ones."
Born William Garson Paszamant in Highland Park, New Jersey, Garson began studying acting at age 13 at the Actors Institute in New York. He made hundreds of appearances on TV and in motion pictures.
Besides Sex and the City, he was perhaps best known as Mozzie, a con man on the TV show White Collar, and also had recurring roles on NYPD Blue, Hawaii Five-0 and Supergirl.
Garson, who was an advocate for adoption agencies, adopted his son, Nathen, in 2009 and marked the adoption in a January Instagram posting that read: "Best day of my life. Always."