Martin Siegel graduated from The University of Texas at Austin and Harvard Law School. After law school, he clerked for the Hon. Irving R. Kaufman on the U.S. Court of Appeals for the Second Circuit. From 1992 to 1994, he was an associate in the Washington D.C. office of Jenner & Block, where he worked on appellate, commercial, intellectual property, and environmental matters, including a habeas petition for death row inmate Kevin Wiggins later granted by the Supreme Court in a decision setting new standards for counsel in capital cases.

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From 1995 to 2000, Siegel served as an Assistant United States Attorney in the Southern District of New York, where his practice focused on bringing civil rights actions, defending statutes from constitutional challenge, and representing  federal agencies and officers. Civil rights cases included a complaint under the Voting Rights Act following fraud in a Bronx school board vote, some of the first cases in the nation under the Freedom of Access to Clinic Entrances Act, a suit targeting discriminatory zoning in violation of the Americans with Disabilities Act, and an investigation of NYC's Parks Department for employment discrimination. In 1999, he received DOJ's Director's Award for his trial defense of Sidney Gottlieb, who devised the CIA's LSD experimentation program in the 1950s.

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In 2000-01, Siegel served as Special Counsel on the staff of the U.S. Senate Judiciary Committee, where his responsibilities included drafting and analyzing legislation on election reform, campaign finance, criminal justice, immigration, and other issues. From 2001-06, Siegel was a partner at Watts Law Firm in Houston, where he worked on commercial, product liability, intellectual property, and other litigation for plaintiffs. 

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In 2007, Siegel opened his own appellate practice. Notable representations included Tolan v. Cotton, 572 U.S. 650 (2014) (reversing dismissal of suit by man wrongfully shot by police); Gimenez v. Franklin County, Wash. (U.S. 2024) (defeating cert. petition constitutionally challenging Washington's Voting Rights Act); Kinney v. Barnes, 443 S.W.3d 87 (Tex. 2014) (permitting websites to remove defamatory content under First Amendment); Connick v. Thompson, 563 U.S. 51 (2011) (representing amicus centers in criminal law and ethics at NYU, Stanford, and other law schools in death row/wrongful imprisonment suit; brief cited by Justice Ginsburg in dissent); GDG Acquisitions v. Govt. of Belize, 849 F.3d 1299 (11th Cir. 2017), and 749 F.3d 1024 (11th Cir. 2014) (representing U.S. company against Belizean government; decisions cited in Restatement (Fourth) of Foreign Relations Law and Wright & Miller’s Federal Practice and Procedure); In re Volkswagen of America, 545 F.3d 304 (5th Cir. 2008) (en banc) (setting circuit standard for motions to transfer venue).

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In 2023, Siegel founded the Appellate Civil Rights Clinic at the University of Houston, which he directed until becoming fulltime faculty at South Texas College of Law.  He is the author of Judgment and Mercy: The Turbulent Life and Times of the Judge Who Condemned the Rosenbergs (Cornell U. Press 2023), a finalist for the National Book Critics Circle’s John Leonard Prize for best first book in any genre. His writing on legal topics has also appeared in the New York Times, the Los Angeles Times, the Houston Chronicle, and law reviews and legal journals.