In 1978, Lamberth became chief of the civil division of the United States Attorney's Office, a position he held until his appointment to the federal bench.

[5] Lamberth presided over Cobell v. Kempthorne, a case in which a group of Native Americans sued the U.S. Department of the Interior for allegedly mismanaging a trust intended for their benefit.

[6] Lamberth, appointed to the bench by President Ronald Reagan, was known for speaking his mind and repeatedly ruled for the Native Americans in their class-action lawsuit.

His opinions condemned the government and found Interior secretaries Gale Norton and Bruce Babbitt in contempt of court for their handling of the case.

After a particularly harsh opinion in 2005, in which Lamberth lambasted the Interior Department as racist, the government petitioned the Court of Appeals to remove him, alleging that he was too biased to continue with the case.

[9][10] On December 29, 2016, Lamberth ordered the preservation of the full classified United States Senate Intelligence Committee report on CIA torture.

[citation needed] In 2010, two federal magistrate judges approved a warrant sought by the Justice Department to search personal e-mails and phone records of Fox News reporter James Rosen related to a story about the North Korean nuclear program.

[13] In August 2010, Lamberth issued a temporary injunction blocking an executive order by President Barack Obama that expanded stem cell research.

[16] Judge Lamberth refused to lift the injunction forbidding the research pending the appeal of his ruling, and the United States Court of Appeals for the District of Columbia Circuit issued an order on September 9, 2010, providing for an emergency temporary lifting of the injunction in the case that had forbidden the research, at the request of the Justice Department.

[24][25] Lamberth has presided over a number of the criminal cases of participants in the January 6 attack on the United States Capitol,[26][27] including Jacob Chansley, the "QAnon Shaman".

[37] In October 2021, Lamberth held D.C. corrections officials in contempt, citing treatment and civil rights abuses of January 6 participant Christopher Worrell.

By 2023, after several hundred participants had been jailed, tried and convicted for their activities, some House Republicans such as Marjorie Taylor Greene sought to characterize them as "political prisoners."