The 33-year-old replaces Lance Blanks, who was fired on April 22 after failing to take the Suns to the playoffs in three seasons.
"Ryan distinguished himself among an impressive group of candidates for our GM position," Suns President of Basketball Operations Lon Babby said Tuesday. "His natural leadership and communication skills will serve the Suns well. And, his prodigious work ethic and ability to identify talent will enable us to take full advantage of the 10 draft choices, including six in the first round, that we have over the next three years. We welcome his championship pedigree to our organization."
McDonough worked his way up from a 23-year-old special assistant to basketball operations in Boston to becoming assistant general manager the past three years.
McDonough has a strong scouting background, working four years in the Celtics' video scouting department, another as director of amateur scouting and as director of international scouting. McDonough spent two seasons as director of player personnel and was promoted to assistant GM in 2010.
As assistant GM, he worked with president of basketball operations Danny Ainge and was principally responsible for draft evaluations of college and international players.
McDonough's father, the late Will McDonough, was a long-time columnist for the Boston Globe. His brother, Sean, is an announcer for ESPN and another brother, Terry, is the former director of player personnel for the Jacksonville Jaguars.
His job in Phoenix won't be easy.
The Suns overhauled their roster during the offseason, trading away two-time league MVP point guard Steve Nash to the Lakers and allowing Grant Hill, another fan favorite, to sign with the Clippers as a free agent.
With nine new players on the roster, Phoenix struggled early in the 2012-13 season and never recovered, finishing 25-57, the second-worst season in franchise history behind the inaugural team, which went 16-66 in 1968-69.