On November 18, 1974, at the age of 38, Flaum was nominated by President Gerald Ford to a seat on the United States District Court for the Northern District of Illinois vacated by Judge Philip Willis Tone. Flaum was confirmed by the United States Senate on December 18, 1974, and received his commission on December 20, 1974. Flaum's service terminated on June 1, 1983, due to elevation to the Seventh Circuit. Flaum was then nominated by President Ronald Reagan on April 14, 1983, to a seat on the United States Court of Appeals for the Seventh Circuit vacated by Judge Robert Arthur Sprecher. Flaum was confirmed by the Senate on May 4, 1983, and received his commission on May 5, 1983. He served as Chief Judge from 2000 to 2006. He remains in active service on the Seventh Circuit. In March 2017, Flaum found that police officers could not be sued for needlessly destroying property during a search because they had prevented the owner from witnessing which officers had caused the damage. Judge David Hamilton partially dissented, arguing that the owner should not have been required to instead plea a novel "conspiracy of silence" claim. On April 4, 2017, Flaum wrote a concurrence when the 7th circuit upheld that employment discrimination based on sexual orientation violates Title VII of the Civil Rights Act. Flaum wrote: "Consequently, employment discrimination based on an employee’s interracial relationship is, in part, tied to an enumerated trait: the employee’s race. This type of discrimination is prohibited by Title VII. The same principle applies here. Ivy Tech allegedly refused to promote Professor Hively because she was homosexual—or a woman who is sexually attracted to women. Thus, the College allegedly discriminated against Professor Hively, at least in part, because of her sex. I conclude that Title VII, as its text provides, does not allow this." Flaum was joined by Kenneth Francis Ripple in his concurrence, and the two of them joined part of the majority opinion written by Diane Wood. On April 19, 2018, Flaum was the deciding vote in blocking Indiana's fetal burial requirement. Flaum was also in the 3–0 majority to block Indiana's ban on abortions due to race, sex, or disability. The majority opinion was written by William Joseph Bauer, and the 2–1 and 3–0 discrepancy comes from the partial dissent of Daniel Anthony Manion. On June 25, 2018, Flaum again cast the decisive vote in favor of abortion rights, to deny rehearing of the April 2018 cases. The U.S. Supreme Court partially overturned and partially affirmed the opinion in Box v. Planned Parenthood of Indiana and Kentucky, Inc. Contrary to his 2018 votes in favor of abortion rights, on November 1, 2019, Flaum voted to rehear a case after a three-judge panel blocked Indiana's parental notification requirements. Flaum joined a dissent written by Michael Stephen Kanne. On September 7, 2018, Flaum ruled that the felon dispossession statutes that barred felons from getting rifles does not violate the Second Amendment. Flaum was joined by Kenneth Francis Ripple over the dissent of Amy Coney Barrett.
