Middleton, 61, holds a 48 percent stake in the franchise. Pete, Jim and Sandy Buck, David Montgomery and Pat Gillick own the remaining 52 percent. Middleton has been a limited partner since 1994, and he has served as chairman of the Phillies' advisory board since '98.
"I'm thrilled with the opportunity," Middleton told MLB.com at the owners' meetings. "It's a privilege and an honor to be asked to be a control person and to have the confidence of my partners, the Bucks, and Major League Baseball and the other owners. I'm looking forward to it. Our fans want us to bring back a championship. We're committed to that. We begin working even harder today than we did yesterday. It's our goal to win a championship."
Montgomery had served as the Phillies' control person and president since 1997. He took a medical leave of absence in August 2014 following jaw bone cancer surgery. Gillick replaced him as president with Andy MacPhail assuming the role following the 2015 season.
Montgomery currently serves as team chairman.
"David's leadership during his tenure was certainly distinguished, and we owe him a debt of gratitude," Middleton said. "Under his leadership, the Phillies won five consecutive division titles, two National League pennants and the 2008 World Series championship."
Phillies' ownership had chosen to stay largely in the background for years until Middleton emerged in the summer of 2015, when the team announced MacPhail would replace Gillick as president. Middleton sat at the dais during the news conference, answering questions and discussing the team's vision for the future.
Middleton has appeared publicly and granted interviews since, asserting himself as the spokesman of the Phillies' ownership.
"It's been a gradual process," Middleton said. "It's probably more of a formal change than anything. I don't know that my life is going to change that much."
"I think the ownership committee has a strong preference that the individuals who are either the largest or one of the largest equity owners in clubs be the individual who is the baseball control person," Commissioner Rob Manfred said. "That makeup, that set of facts, is most consonant with the concept of the control person that's embedded in our ownership guidelines."
Middleton graduated from Amherst College in 1977 with a bachelor's degree. He graduated from Harvard Business School in '79 with a master's degree in business administration.
Middleton and his wife, Leigh, received the Philadelphia Award in 2013 in recognition of their philanthropic endeavors. He is involved with organizations that include Project HOME, Amherst College, Dartmouth College, the Philadelphia Museum of Art, Penn Medicine, Teach for America and the Bryn Mawr Presbyterian Church. He has provided support for career and technology education in the city of Philadelphia. Middleton also serves as a member of the board of trustees of Amherst College.