Sam Callahan focuses on appellate and complex civil litigation in state and federal courts. He has successfully argued cases in multiple federal courts of appeals; represented parties at the certiorari and merits stages in the US Supreme Court; and briefed cases raising high-stakes issues of constitutional interpretation, administrative law, and criminal procedure, among others. Mr. Callahan has significant experience with political and voting-rights litigation, having played key roles in successful challenges to North Carolina’s gerrymandered election districts and to the addition of a citizenship question to the 2020 Census.

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.


  • Trade association in successful DC Circuit challenge to agency rule imposing emissions and fuel-consumption regulations on commercial trailers. Truck Trailer Manufacturers Association, Inc. v. EPA, 17 F.4th 1198 (D.C. Cir. 2021).
  • North Carolina voters in successful challenge to partisan gerrymandering of state’s 2021 congressional and legislative maps. Secured landmark decision from North Carolina Supreme Court holding that partisan gerrymandering violates the state’s constitution. Harper v. Hall, 867 S.E.2d 554 (N.C. 2022). Successfully defended state court’s ruling against emergency stay application in US Supreme Court. Moore v. Harper, No. 21A455 (Mar. 7, 2022).
  • Major property developer in securing unanimous affirmance of agency decision authorizing a $200 million community redevelopment project in Georgetown. DC Preservation League v. Mayor’s Agent for Historic Preservation, 236 A.3d 373 (D.C. 2020).
  • Public health researcher in briefing and arguing Second Circuit appeal of defamation suit targeting statements criticizing a company’s scientific research and marketing. Secured unanimous decision affirming dismissal on ground that client’s statements were protected opinions under New York law. Valley Electronics AG v. Polis, No. 21-2108 (2d Cir. Mar. 28, 2022).
  • Criminal defendant in briefing and arguing successful Fourth Circuit appeal asserting ineffective assistance of trial counsel. Convinced Fourth Circuit to unanimously vacate the dismissal of a § 2255 petition and to order evidentiary hearing regarding failure to seek suppression of un-Mirandized statements. United States v. Pressley, 990 F.3d 383 (4th Cir. 2021). On remand, first-chaired evidentiary hearing, persuaded district court to vacate client’s convictions and life sentence, and secured favorable plea agreement leading to client's release from custody.
  • Dialysis provider in successful Ninth Circuit appeal involving Medicare statutes. Secured pathbreaking decision holding that medical service providers can sue health plans for discriminating against patients needing dialysis. DaVita Inc. v. Virginia Mason Memorial Hospital, 981 F.3d 679 (9th Cir. 2020).
  • Major cruise lines in securing dismissal of over a dozen high-profile federal lawsuits related to COVID-19 exposure on cruise ships. Developed successful strategy for moving to strike jury demands and removing to federal court in cases presenting novel issues under maritime law.
  • Healthcare service provider in Eleventh Circuit appeal of private antitrust lawsuit under the Clayton Act. Secured unanimous affirmance of dismissal for lack of antitrust standing.
  • Pharmaceutical industry trade group in multiple pending constitutional challenges to state laws related to drug pricing.
  • Veteran at US Supreme Court in pending case involving Congress’s ability to authorize suits against states pursuant to its War Powers. Torres v. Texas Dep’t of Public Safety, No. 20-603 (cert. granted).
  • Immigration-rights organizations at US Supreme Court in successful Administrative Procedure Act challenge to the attempted addition of a citizenship question to 2020 Census. Department of Commerce v. New York, 139 S. Ct. 2551 (2019).
  • The State of Oklahoma at US Supreme Court in case challenging the State's ability to prosecute major crimes on former Indian lands in eastern Oklahoma. McGirt v. Oklahoma, 140 S. Ct. 2452 (2020).
  • Former Members of Congress and House General Counsels as amici curiae in several cases at the US Supreme Court and in federal appellate courts involving separation of powers, including Trump v. Mazars USA, LLP, 140 S. Ct. 2019 (2020); Trump v. Sierra Club, No. 20-138; and Committee on the Judiciary of the US House of Representatives v. McGahn, 968 F.3d 755 (D.C. Cir. 2020) (en banc).


  • JD, Harvard Law School, 2016, magna cum laude
  • BM, Trumpet Performance and BA, Economics, Indiana University, 2013, Phi Beta Kappa
  • District of Columbia
  • Maryland
  • US Court of Appeals for the First, Second, Third, Fourth, Fifth, Seventh, Ninth, Eleventh, 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
  • Music Grant Panelist, National Endowment for the Arts

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