The team said the sale closed after a California court confirmed the authority of Shelly Sterling, on behalf of the Sterling Family Trust, to sell the franchise to the former Microsoft CEO. Her estranged husband, Donald Sterling, had unsuccessfully fought the sale of the team he owned since 1981 in court.
The NBA Board of Governors had previously approved the $2 billion sale.
"I am humbled and honored to be the new owner of the Los Angeles Clippers," Ballmer said in a statement. "Clipper fans are so amazing. They have remained fiercely loyal to our franchise through some extraordinary times."
The Christian Science Monitor reported in late May that Ballmer had won the bidding war for the Clippers for a record-setting deal.
The potential $2 billion sale would dwarf the record $550 million purchase of the Milwaukee Bucks last month, which had followed the then-record $534 million paid for the Sacramento Kings last year. According to reports, Ballmer beat out a $1.6 billion offer by a group of investors headed by media moguls David Geffen and Oprah Winfrey, as well as a $1.2 billion bid by a group of Los Angeles billionaires who partnered with former NBA star Grant Hill.
Adam Streisand, Ballmer's attorney, said Tuesday that Superior Court Judge Michael Levanas signed the order authorizing the sale even if Donald Sterling's attorneys filed an appeal.
"We were ready," Streisand said. "Within minutes, the deal was signed, sealed and delivered."
He said even if Donald Sterling seeks an emergency order directing the judge to vacate his order, the attorney is confident an appellate court would agree that Levanas made the correct decision.
Donald Sterling's attorneys weren't immediately available to comment.
The transaction ends Donald Sterling's run as the longest-tenured owner in the NBA after 33 years. After buying the Clippers in 1981, he moved the franchise from San Diego to Los Angeles three years later.
The 80-year-old real estate mogul has been in probate court fighting his wife's proposed deal to sell. At issue was whether Donald Sterling killed the deal by revoking the trust after his wife removed him as a trustee. Shelly Sterling acted after doctors found Donald had symptoms of Alzheimer's disease.
The drama began in April when a recording surfaced of Donald Sterling dressing down his young girlfriend for bringing black men to Clippers games. The audio spurred the NBA to ban Sterling for life and fine him $2.5 million.
His wife of 58 years then took control of a family trust and negotiated a record $2 billion sale of the team to Ballmer. Shelly Sterling said she was initially given her husband's blessing to sell the team and he praised the deal she reached.
When it came time to sign it at the end of May, however, Sterling said he would not sell and would sue the league.
Ballmer said he will be "hardcore" in giving the team, coach Doc Rivers, the staff, and players the support they need.
Rivers called it "an amazing new day in Clippers history," and said he's inspired by Ballmer's passion for the game.
As reported by The Christian Science Monitor in June, this is not Ballmer's first attempt to buy a NBA team.
Ballmer and investor Chris Hansen headed a group that agreed to a deal to buy the Kings from the Maloof family in January 2013 with the intention of moving the team to Seattle, where the SuperSonics played until 2008.
But Sacramento Mayor Kevin Johnson lobbied the NBA for time to put together a bid to keep the team in California, and though the Ballmer-Hansen group later increased its offer, owners voted to deny the bid for relocation and the Kings were sold to Vivek Ranadive.
Ballmer, Rivers and Clippers players will attend a new fan festival announced for next Monday at Staples Center.