"The Court's decision could hardly have been more thorough, fact-based, and well-reasoned", David McAtee, AT&T's general counsel, said in a statement. "We are ready to defend the (lower court ruling) at the DC Circuit Court of Appeals".
By opposing the merger, the government was forced to argue against a legal doctrine which favours vertical mergers - which is a combination between companies which do not compete directly with each other. It's a very interesting and unexpected outcome but one to keep a watchful eye on going forward.
For instance, it will manage the Turner networks as part of a separate business unit, distinct from operations of AT&T Communications.
The department's antitrust division filed a notice of appeal Thursday in Washington federal court.
Area MPP leading Tory rollback of Ontario sex-ed curriculum
The 2015 curriculum introduced students to concepts that did not exist in 1998, such as "sexting" and online bullying. Some parents, too, said they would have to make adjustments in light of the change.
AT&T Chief Executive Randall Stephenson told reporters on Thursday at a conference in Sun Valley, Idaho, that the company was not surprised about the Justice Department's decision to appeal.
A spokesman for the Justice Department declined to comment on the filing.
Mr Trump has publicly feuded with Time Warner's CNN, calling it "failing" and a purveyor of "fake news".
AT&T is a phone, cable and satellite company, and the biggest pay TV provider in the U.S. - claiming about 25 million of the approximately 90 million USA households that are pay TV customers.
Federal lawyers contended that the combined company would be too large and powerful, allowing it to run roughshod over other players in the entertainment and media world and to hike prices on consumers. That apparently wasn't enough, however, to satisfy the Justice Department.
The Trump administration had opposed the merger, moving to block it on grounds that it hurt media industry competition. Waiting in the wings are potential big-billions deals involving Verizon and CBS and T-Mobile and Sprint.