The acquisition means AT&T will be the nation's top pay-TV distributor.
Initially AT&T and Time Warner planned to use a "selective enforcement" defense, alleging that the administration was blocking the deal because of Trump's dislike of CNN.
Leon - who presided over the Justice Department's suit against Comcast's acquisition of NBCUniversal that was settled with a consent decree - signaled his time table to announce his decision on AT&T for Tuesday at 4 p.m., giving the company and the Justice Department time to settle the case. He also called their arguments against the deal "gossamer thin" and "poppycock".
Judge Leon has said he will issue his decision on AT&T's proposed deal today.
With this merger, the amount of companies that own everything we watch, see, and hear is shrinking down even more. The ruling comes one day after the repeal of net neutrality, meaning that AT&T now has more freedom to prioritize the delivery of select content on its network.
But apart from the federal government, states could also challenge this, something the two companies have acknowledged. Those companies started making their own programming in just the last several years, but they now spend billions of dollars a year on it, and users can stream their video on apps in homes and on mobile devices, pulling attention from traditional media businesses.
BREAKING: AT&T, Time Warner Merger Cleared
"The government here has taken its best shot to oppose this merger, " Leon said, speaking to a packed courtroom in an unusual session weeks after the trial ended.
The Justice Department rarely sues to block a "vertical merger" such as the AT&T and Time Warner because they are not considered as economically risky as "horizontal mergers" - or ones in which a companies buys direct competitors such as T-Mobile's pending deal for Sprint.
Makan Delrahim, the Justice Department's antitrust chief, said prosecutors were "disappointed" with the decision but did not reveal whether they would seek a stay or appeal the ruling.
The judge's 200-page opinion permits the merger to go forward with no conditions. AT&T pushed to merge with Time Warner to better compete with these newer companies and claim back some of the market.
AT&T applauded the court's decision.
In making its case, the US government argued that the merger of AT&T could harm consumers in a number of ways.
The government estimated costs to industry rivals, such as Charter Communications Inc, would increase by $580 million a year if AT&T owned Time Warner.
USA district judge Richard Leon sided with AT&T, approving the deal without conditions.
In May it was revealed that AT&T had paid Michael Cohen, Trump's personal attorney, $600,000 for consultancy fees as it attempted to woo the White House.
Belmont Stakes: Justify completes Triple Crown with wire-to-wire win
Justify was the Belmont's wire-to-wire leader, jumping to a lead out of the gate and never relinquishing it. Justify won the 150th running on the Belmont Stakes on Saturday, becoming the 13th Triple Crown victor .