From 1975 to 1987, Trenga was in private practice in Washington, D.C. with the law firm Sachs, Greenebaum & Tayler, becoming a partner in 1982. He was managing partner at Hazel & Thomas in Alexandria, Virginia from 1987 to 1998, and back in the District of Columbia from 1998 to 2008 with Miller & Chevalier.
In October 2009, Trenga set aside the jury conviction of two top salespeople at Teach Me to Trade, a part of Whitney Information Network, which uses infomercials and hotel seminars across the country to sell courses and software on making money in the stock market. In a 51-page ruling, Trenga said prosecutors failed to show Utah residents Linda Woolf and David Gengler had been part of any fraud scheme. On September 24, 2012, the American Tradition Institute Environmental Law Center1 filed a lawsuit against the United States Environmental Protection Agency in the U.S. District Court and asked the court to find that the EPA failed to comply with laws controlling human experimentation. The ATI sought to “immediately halt EPA’s human experimentation which intentionally exposes human subjects, to ‘fine particles’ such as those ‘produced by car and coal-fired power plants.’” In addition, the ATI sought declaratory relief, asking the court to find that the EPA failed to adequately inform participants that the pollution they would be inhaling as part of the experiment posed a serious risk to their health. On January 31, 2013, the ATI’s case against the EPA was thrown out by Judge Anthony Trenga who determined the EPA’s alleged actions did not constitute “final agency action” under the Administrative Procedure Act.10 Judge Trenga also determined that because the ATI was not personally being harmed by the experiments, it did not have standing to pursue the case. On March 24, 2017, Trenga was the first federal judge to rule in favor of the Trump administration's executive order that limits travel from six Muslim-majority countries. On May 16, 2019, Trenga re-incarcerated whistleblower Chelsea Manning, holding her in contempt after she refused to testify before a federal grand jury, adding the threat of daily fines up to $500 after 30 days and $1,000 after 60 days. This came a week after Manning was freed from 62 days of incarceration for the same offense before another federal grand jury. On September 4, 2019, Trenga ruled that the United States government’s watchlist of “known or suspected terrorists” violates the constitutional rights of those listed on it. On September 24, 2019, Trenga set aside the conviction of Bijan Khan, business partner of Donald Trump's former National Security AdvisorMichael Flynn, on acting as an agent for a foreign power without notifying the Justice Department. Flynn was found guilty and was awaiting sentencing in his own foreign power lobbying case. On March 12, 2020, Trenga freed whistleblower Chelsea Manning, who had attempted to kill herself the day before. Trenga ordered that Manning pay $256,000 in fines that had accumulated over the course of her confinement.
