"The (lower) court's decision could hardly have been more thorough, fact-based, and well-reasoned", said a statement from David McAtee, AT&T's general counsel. The Justice Department had 60 days to appeal the merger after it was approved.
In addition, AT&T Communications will have no say in setting Turner's prices or other terms in contracts with companies that distribute its content.
The logos for Time Warner and AT&T appear in June above trading posts at the New York Stock Exchange.
The government's notice of appeal filed in U.S. District Court in Washington did not disclose on what grounds it intends to challenge the approval.
AT&T shares slipped 1% in after-hours trade in NY.
AT&T countered that a combined company would be better able to compete with a growing roster of online video competitors including Facebook, Google, Netflix and Apple.
The company did not immediately comment on the DoJ move.
At the time of his ruling, the judge wrote that he did not believe "the government has a likelihood of success on the merits of an appeal" were it to consider one.
In 2016, the Justice Department had demanded that AT&T sell the Turner networks, which include CNN, as part of approving the merger.
Earlier this month, AT&T raised the cost by $5 per month for its digital streaming offerings for its satellite service DirecTV, called DirecTV Now. The day after Leon delivered his ruling, Comcast made a $65 billion offer for 21st Century Fox, in an attempt to outbid Disney for Fox's entertainment assets. The DOJ did not, but left open the question of whether it would still appeal. Trump has publicly feuded with Time Warner's CNN, calling it "failing" and a purveyor of "fake news". Still, that issue could arise during the appeal. In its lawsuit and during the trial, Justice Department attorneys said the combined companies would have the power to strong-arm competitors and drive up prices paid by customers. The appeals process could take many months, leaving lingering uncertainty over AT&T's plans.