Simpson serves on the House Appropriations Committee. He is Chairman of the House Interior and Environment Subcommittee and also serves on the House Energy and Water Development Subcommittee as well as the Labor, Health and Human Services, Education Subcommittee. These committees have jurisdiction over funding for several agencies and programs critical to Idaho including the Department of Energy, the Department of the Interior, the Forest Service, the National Parks Service, the National Endowment for the Arts, the Smithsonian Institute, and the Department of Labor.

Simpson is one of the House's leading advocates for a new energy policy and a renewed commitment to research and development of improved nuclear energy technologies. Simpson has also gained national attention for his bill to split the massive, overburdened 9th Circuit Court of Appeals as well as his Sawtooth National Recreation Area and Jerry Peak Wilderness Additions Act which addresses the concerns of economic growth and stability for rural Idaho and resolves long time wilderness debate over the Boulder-White Clouds and was signed into law in August of 2015. Simpson has long championed a fix to fire borrowing, having authored the Wildfire Disaster Funding Act. Simpson's leadership led to the passage of the FY18 Omnibus Spending Bill that provided historic forest management reforms, most notably treating wildfires like other natural disasters for budgetary purposes. Simpson also authored the framework for the Great American Outdoors Act, which was signed into law on August 4, 2020. The legislation secured permanent funding for the existing Land and Water Conservation Fund (LWCF), which has been a historically important program to conserving public lands and improving outdoor recreation opportunities. In the 116th and 117th Congresses, Simpson has also championed the Farm Workforce Modernization Act (FWMA), a common-sense workforce solution for American agriculture. The bill has support from 300 agricultural groups, over 40 of which are from Idaho. This legislation makes meaningful reforms to the H-2A agricultural guest worker program and will provide economic stability to our rural communities while making our country safer by creating a legal, merit-based agriculture immigration system that allows our valuable law enforcement resources to focus on the bad actors. The FWMA has passed twice in the House of Representatives.

His political career began in 1980, when he was elected to the Blackfoot City Council. In 1984, he was elected to the Idaho Legislature where he served until 1998, the last six years serving as Speaker. Simpson was born in Burley, Idaho and raised in Blackfoot. He graduated from Utah State University and earned his DMD from Washington University School of Dental Medicine in St. Louis, Missouri. After graduation, he joined his father and uncle at the Simpson Family Dental Practice in Blackfoot.

Simpson married his high school sweetheart, Kathy, and they have been married for over 50 years. He is an avid golfer and reader who also enjoys painting and experiencing the exceptional scenery Idaho has to offer, with Kathy and their dog Charley.