William McClellan "Mac" Thornberry is an American politician who served as the U.S. Representative for Texas's 13th congressional district from 1995 to 2021. A member of the Republican Party, Thornberry represented the most Republican district in the United States by partisan voting index. The district covers the Texas Panhandle and stretched between the Oklahoma and New Mexico borders.

Thornberry is a lifelong resident of Clarendon, 60 miles (97 km) east of Amarillo in the heart of the 13th. His family has operated a ranch in the area since 1881. He received his Bachelor of Arts in history from Texas Tech University in Lubbock. He then obtained his Juris Doctor from the University of Texas School of Law in Austin.

He served as a staffer to two other Texas Republican congressmen, Tom Loeffler and Larry Combest, and as deputy assistant Secretary of State for Legislative Affairs under Ronald Reagan before joining his brothers on the family ranch. Thornberry has called President Reagan "...a great man and a great president, ranking in the top tier of all of our chief executives." He also practiced law in Amarillo.

From 2015 to 2019, Thornberry served as chairman of the House Armed Services Committee, the first Texan of either party to hold this position. The committee oversees the Pentagon, all military services, and all Department of Defense agencies, including agency budgets and policies.

Since taking the committee gavel at the beginning of the 114th Congress, Thornberry has spearheaded a major Department of Defense acquisition reform effort that has received bipartisan and bicameral support from House Armed Services Committee Ranking Member Adam Smith (D-Wash.) and Senate Armed Services Committee Chairman John McCain (R-AZ).

Thornberry previously served on the United States House Permanent Select Committee on Intelligence.