Sam Callahan focuses on appellate and complex civil litigation. He has represented individuals, businesses, and governments in the US Supreme Court and federal appellate courts, and has briefed and argued issues ranging from administrative law to sovereign immunity to novel questions of criminal law and procedure.

Mr. Callahan joined the firm in 2018 after serving as a law clerk to the Honorable Paul V. Niemeyer of the United States Court of Appeals for the Fourth Circuit and the Honorable Christopher R. Cooper of the United States District Court for the District of Columbia. He is a magna cum laude graduate of Harvard Law School, where he served as an Executive Editor of the Harvard Law Review.


  • Immigration-rights organizations at the US Supreme Court in a successful challenge to the attempted addition of a citizenship question to the 2020 Census. Department of Commerce v. New York, 139 S. Ct. 2551 (2019).
  • The State of Oklahoma at the US Supreme Court in a case challenging the State's ability to prosecute major crimes on former Indian lands in eastern Oklahoma. McGirt v. Oklahoma, No. 18-9526 (pending).
  • DaVita Inc. in Ninth Circuit appeals involving a Medicare statute that prohibits discrimination by employer health plans.
  • Truck Trailer Manufacturers Association at the DC Circuit in a challenge to novel emissions and fuel-consumption regulations.
  • Voters and political groups in successful partisan gerrymandering challenge to North Carolina's state legislative districts. Helped draft post-trial briefing that led to landmark decision under North Carolina constitution. Common Cause v. Lewis, No. 18-cvs-14001 (N.C. Super. Ct.).
  • Former Members of Congress and House General Counsels as amici curiae in several cases at the US Supreme Court and federal appellate courts involving the separation of powers, including Trump v. Mazars, No. 19-715 (pending).


  • JD, Harvard Law School, 2016, magna cum laude
  • BM/BA, Trumpet Performance and Economics, Indiana University, 2013, Phi Beta Kappa
  • District of Columbia
  • Maryland
  • US Court of Appeals for the First, Second, Third, Fourth, Fifth, Seventh, Ninth, and DC Circuits
  • US District Court, District of Columbia
  • US District Court, District of Maryland
  • US Court of Appeals for the Fourth Circuit, The Honorable Paul V. Niemeyer
  • US District Court, District of Columbia, The Honorable Christopher R. Cooper

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