The case had been closely followed as a bellwether for competition regulation in the U.S., where there is growing concern that a small number of companies dominate many sectors.
"We are ready to defend the Court's decision at the D.C. Circuit Court of Appeals", AT&T General Counsel David McAtee said in a statement.
Leon also wrote that if the DOJ appealed and won, the Circuit Court would have no qualms about unwinding the merger.
AT&T shares fell one per cent after the bell. Since taking office, Trump has not commented on the deal itself, but in an exclusive interview with Breitbart News past year he showed amenability to a more aggressive stance on enforcing competition in the media world.
AT&T completed the $81bn (£60bn) deal shortly after.
First video shows boys in hospital after cave rescue
On social media, Mr Volanthen has been described as a "real hero" with many calling for him and Mr Stanton to be honoured. The first images have been released of the Thai soccer team recovering in the hospital after being rescued from a cave.
Soon after the deal closed, AT&T announced a price increase for DirecTV Now, the company's online streaming video service. The company said it was increasing rates to bring the product in line with the rest of the market. But it is unclear how that price hike could affect the litigation.
Leon rejected all of the government's arguments against the deal, one of the largest media mergers ever.
Speaking to reporters at the Sun Valley conference in Idaho, AT&T CEO Randall Stephenson said his company was expecting the Justice Department to appeal, but are prepared and "not worried" about it.
The appeal could be seen as a warning sign for companies that have pursued their own mergers in the wake of Leon's decision, Kimmelman added.
No matter how the Justice Department's appeal unfolds, AT&T's completed acquisition of Time Warner could prove hard to untangle. The Justice Department had agreed not to seek an emergency court order preventing the deal from closing after AT&T promised to operate Time Warner's Turner Broadcasting as a separate business unit until 2019.