DiBenedetto's four-man group had struck a deal with UniCredit bank in April to take a 67 percent share of the club, with the Americans controlling 60 percent of the new consortium.
UniCredit, who retain the remaining 40 percent, had last year taken on the Sensi family's majority share in the club in return for cancelling their debt.
"We have concluded all the formalities, the price has been paid and the equity distributed," Roberto Cappelli, the UniCredit lawyer who is acting as Roma's interim president, said in a statement on the club's website.
"The operation has lasted nine months, like a difficult pregnancy, but we have finally reached our goal.
"The intention is not to deceive but to build something that will last," added Cappelli, who is expected to be replaced as Roma president by 62-year-old DiBenedetto at the club's next board meeting in September.
"This is why UniCredit did not wish to accept the first offer that was made but to wait and choose the best project for the club."
DiBenedetto, who is also a partner in New England Sports Ventures - which owns baseball club Boston Red Sox and Premier League side Liverpool, heads a group comprised of investors James Pallotta, Michael Ruane and Richard D'Amore.
Despite the protracted takeover, Roma, who finished a disappointing sixth in Serie A last season, have recruited fledgling Spanish coach Luis Enrique from Barcelona B and experienced Walter Sabatini as sporting manager.
