Following private practice, Immergut served as an assistant United States attorney for the Central District of California in Los Angeles for six years.

[3] Immergut served for five years as a deputy district attorney in Portland, Oregon, where she primarily prosecuted white collar crimes.

In 1998, Immergut was a Multnomah County deputy district attorney when she went to work for Independent Counsel Kenneth Starr, who was investigating then-President Bill Clinton.

Serving two years in the position, she prosecuted cases involving white collar crime and worked on Project Safe Neighborhoods, a national gun violence reduction initiative.

She managed a staff of approximately 107 people, including 51 assistant U.S. attorneys, who handled civil litigation on behalf of the United States and criminal investigations and prosecutions involving violations of federal law such as white collar crime, narcotics trafficking, violent crime, money laundering and cybercrime.

[6] In January 2008, Immergut applied to succeed Judge Garr King on the United States District Court for Oregon.

[5] But after news reports highlighting her role in the investigation of President Bill Clinton's sex scandal, she was not one of the final candidates for the position, which ultimately Marco A. Hernandez was appointed to.

[16] Immergut's ruling held that the rule unlawfully "excludes female competitors from the only available professional soccer opportunity in the United States because they are under 18, regardless of talent, maturity, strength, and ability" and, pointing out that Major League Soccer did not have a similar age limit, that "the balance of equities and the public interest strongly favor affording girls in the United States the same opportunities as boys.

"[17] On March 10, 2023, Immergut ruled that Salem police officer Robert Johnston had no way of knowing he violated Eleaqia McCrae's rights when he shot her with rubber bullets at a protest in 2020, a legal principle known as qualified immunity.

[20] On July 14, 2023, Immergut upheld Oregon's gun control law, Measure 114, "banning large capacity magazines and requiring a permit to purchase a gun falls in line with “the nation’s history and tradition of regulating uniquely dangerous features of weapons and firearms to protect public safety.