He served as a law clerk for Judge Edward Weinfeld of the Southern District of New York and then for Justice Potter Stewart of the United States Supreme Court.

[2] Koeltl is known for his October 2006 decision to sentence civil rights lawyer Lynne Stewart to 28 months in prison for providing material assistance to a terrorist, her client, 1993 World Trade Center bombing mastermind Omar Abdel-Rahman, by secretly passing messages to his radical followers in Egypt.

[3] The Second Circuit Court of Appeals ordered Koeltl to reconsider whether that sentence was too light and to take into account the government's arguments that she had committed perjury at her trial and abused her position as a lawyer.

[4][5] In 2011, he presided over the case involving Raffaello Follieri, who pleaded guilty to conspiracy, fraud and money laundering in connection with purchases of property from the Catholic Church.

The Follieri case received significant media scrutiny due to his relationship with celebrities, notably Anne Hathaway and several politicians, including former president Bill Clinton[6] and 2008 Republican presidential nominee John McCain.

[10] In April 2018 Koeltl was assigned to preside over a civil lawsuit filed by the Democratic National Committee against the Russian Federation, WikiLeaks, the Donald Trump presidential campaign, and several individuals.

[11] Koeltl, in dismissing the suit in July 2019, described WikiLeaks' publishing activities as "plainly of the type entitled to the strongest protection that the First Amendment offers.

"[12] In September 2020 Koeltl presided over Sam Party v. Kosinski in which the Serve America Party sued Todd D. Valentine and Robert A. Brehm, the Co-Execute Directors of the New York State Board of Elections, and Peter S. Kosinski, Douglas A. Kellner, and Andrew J. Spano, the Commissioners of the New York State Board of Elections (NYSBOE).