Mr. Tillman was appointed chairman of the Marathon Oil board of directors in 2019, in addition to his responsibilities as president and CEO. He became a director, president and CEO of Marathon Oil in 2013.

Prior to joining Marathon Oil, he served as vice president of engineering for ExxonMobil Development Company (a project design and execution company), where he was responsible for all global engineering staff engaged in major project concept selection, front-end design and engineering. He served as North Sea production manager and lead country manager for subsidiaries of ExxonMobil in Stavanger, Norway, from 2007 and 2010, and as acting vice president, ExxonMobil Upstream Research Company from 2006 to 2007. Mr. Tillman began his career in the oil and gas industry at Exxon Corporation in 1989 as a research engineer and has extensive operations management and leadership experience that has included assignments in Jakarta, Indonesia; Aberdeen, Scotland; Stavanger, Norway; Malabo, Equatorial Guinea; Dallas and New Orleans.

He is a board member of the Houston American Heart Association, Southwest Region American Heart Association, American Petroleum Institute, and American Exploration and Production Counsel. He is also a member of the Engineering Advisory Council of Texas A&M University, National Petroleum Council, and the Society of Petroleum Engineers. Mr. Tillman is an emeritus board member of Spindletop Charities and serves as a member of the Celebration of Reading Committee within the Barbara Bush Houston Literacy Foundation.

Mr. Tillman holds a B.S. in chemical engineering from Texas A&M University and a Ph.D. in chemical engineering from Auburn University.