In 1985, during the Reagan administration, he was appointed as an Assistant Attorney General in the Office of Legal Counsel, United States Department of Justice, the office responsible for providing legal opinions and informal advice to the White House, the Attorney General, and Executive Branch Departments and Agencies on issues of statutory, regulatory, constitutional and international law.
[8] He was Meese's chief aid in the so-called weekend investigation that discovered Lt. Col. Oliver North's plan to divest funds from the CIA's covert sale of missiles to Iran to the Nicaraguan Contras.
[10] Cooper led the legal team for the defendant-intervenors in Hollingsworth v. Perry, defending California Proposition 8 in 2008, which banned same-sex marriage in the state.
[13] During this process, a self-professed left leaning registered Democrat and Biglaw partner explained his reasons for supporting Cooper.
[14] He described Cooper as “thoroughly devoted to drawing principled constitutional lines,” “an immovable rock” for upholding the rule of law, and “one of the most principled lawyers you’ll ever encounter.”[15] Cooper & Kirk was described by The National Law Journal in 2021 as "one of the most influential firms in Washington", and "a player in many of the major events in American history over the past quarter-century".
[16] Alumni of the firm Cooper founded include U.S. senators such as Ted Cruz and Tom Cotton;[14] federal judges such as James Ho,[16] Victor J. Wolski,[17] and Howard C. Nielson Jr.;[18] and solicitor general Noel Francisco.
[19] In 2022, legal commentator David Lat wrote that Cooper & Kirk may have the "most impressive and influential alumni" of any law firm, especially adjusting for size.
On May 16, 2022, by a vote of 6-3, in an opinion written by Chief Justice John Roberts, the Supreme Court ruled in favour of Ted Cruz.
[25] The substantive question in that case was whether the Equal Protection Clause of the Fourteenth Amendment prohibits the state of California from defining marriage as the union of one man and one woman.
Cooper represented the proponents of Proposition 8, a ballot initiative adopted by the voters of California that defined marriage as being between one man and one woman.
[25] In this case, the Supreme Court considered whether Alabama's franchise tax discriminates against interstate commerce, in violation of the Commerce Clause, and whether the Alabama Supreme Court's refusal to permit the South Central Bell Telephone Company and others to raise their constitutional claims because of res judicata deprives them of the due process of law guaranteed by the Fourteenth Amendment.
[31] The question in that case was whether the inclusion of clergy who offer prayers at official public school ceremonies violated the Establishment Clause of the First Amendment.