Mississippi State, where Hull played his college ball, announced he passed away on Tuesday afternoon. The cause of death was not immediately known.
Hull was a three-time Pro Bowl selection during 11 seasons with the Bills. He was credited for playing a key role in helping run the team's no-huddle "K-Gun" offense. The Bills honored Hull in 2002 by placing his name on Ralph Wilson Stadium's Wall of Fame.
"We have suffered a tremendous loss," Bills Hall of Fame running back Thurman Thomas wrote in an email to The Associated Press. "Kent was a teammate, big brother and best friend. My heart breaks for His amazing family. My heart breaks period. He will be a best friend and in my heart forever."
Kelly described Hull as a "man of great courage and character," in a statement released by the Bills.
"Words fall short when the pain runs so deep," Kelly said. "I'm finding it hard to describe what this loss means to me and my family. Kent Hull was my best friend. ... He will be missed and never forgotten."
Hull was inducted into Mississippi State's sports hall of fame in 2000, and then the state's sports hall of fame three years later.
Hull broke into pro football in 1983 with the USFL New Jersey Generals, where he eventually was joined by Kelly in 1986, just before the league folded. The two then headed to the Bills, the team that still held the rights to Kelly after selecting him in the first round of the 1983 draft.
They became part of the core of a team that ushered in the Bills' golden era. Hull was a fixture on the offensive line of a team that made four straight Super Bowl appearances -- from 1991 to 1994 -- only to lose them all.
The Bills ranked first in the NFL in yards rushing in both 1991 and '92, and finished in the top six in total yards from 1989 to 1993.
Hull also joined Kelly in announcing his retirement following the 1996 season.
When his playing days were over, Hull actively worked his cattle farm in northeastern Mississippi.
He was tough and durable as a player, and at one point appeared in 121 consecutive games with the Bills. He appeared in 189 games in all with Buffalo.
His sudden death left his former teammates, Hall of Fame coach Marv Levy and even their respective families inconsolable.
"I'm so, deeply saddened," Levy said. "I coached for 47 years, and he honestly was one of the very most memorable individuals who was my privilege to coach."
Aside from crediting Hull's quick-thinking ability to help run the offense, Levy praised the player for being a great teammate and leader.
"He was an enjoyable man to be around. He made life so pleasant for his teammates, or who ever he was with," Levy said. "He represented everything that sports should be."
Levy's wife, Fran, was so heartbroken she sobbed openly upon learning the news. Thomas' wife, Patti, also had difficulty speaking because she was overcome by emotion when reached by phone.
Hull was a bear of a man, who was both outgoing, funny and philosophical. During Kelly's induction into the Pro Football Hall of Fame in 2002, Hull commented on the sea of Bills fans in attendance at Fawcett Stadium and said: "I feel like I'm in Buffalo. Look at this."
Following his last game, a 30-27 playoff loss to Jacksonville, Hull remarked how opportunistic the Jaguars were.
"They seemed like the team that we used to be," Hull said.
Kelly paid tribute to his center when Hull retired.
"Not only was he a great competitor and a great player, he was one of my best friends," Kelly said then. "He's a guy that if I ever wind up having a boy, I'd hope he grow up to be just like Kent Hull. He's a family guy and a person I love very much. I'm going to miss him."
At Mississippi State, Hull was part of one of the Bulldogs' most memorable games, when they pulled out a 6-3 victory over top-ranked Alabama in 1980. Hull played a critical role in the Bulldogs' option attack which set numerous school records for yards rushing and total offense, some of which still stand today.
Hull is survived by his wife, Kay, and two children, Drew and Ellen. Funeral arrangements had not yet been announced.