United agreed to sell Smalling to Roma for a fee rising to £18.1million after the defender made it clear he had no interest in staying at the club this season.
"It all began in July 2010, and now 10 years, 323 appearances, 2 titles, and 6 cups later it has come to an end," Smalling wrote on Tuesday morning. "Utd is a special place, and we achieved special things together, something I am beyond proud about. An unrivalled culture, where winning is not a want it is a need.
"I leave not only a better player, but a better person, with Utd forever in my DNA. For that I have to thank all the staff, everyone connected to the club, and you the fans. Nothing surer you will continue to greatness."
As well as his farewell message to United the English centre-back followed up his tweets with another one expressing his delight having finally sealed a return to Roma.
He continued: "I arrived last summer, and although temporary at the time, I knew instantly it would be my permanent home. Cannot wait to get started again, the journey continues, enjoy the ride!"