After graduation, Sheffield began his career as a production and reservoir engineer for
Amoco Production Co. He subsequently joined Parker & Parsley as a petroleum engineer in 1979 and was promoted to vice president-engineering in September 1981. In April 1985, he was elected president and director, and on January 19, 1989, he became chairman of the board and CEO. Between 2010 and 2014, Sheffield helped Pioneer Natural Resources in raising billions of dollars from investors in India, China, and Wall Street. Upon his return, he implemented new return of capital framework for the exploration and production industry; reducing capital spending, scaling back drilling plans, and increasing returns to investors through dividends and share buyback schemes. Sheffield retired the end of 2023 with Richard Dealy succeeding him. Sheffield is on the advisory board of
L1 Energy and the board of directors of The Williams Company, Inc. Since 2020, Sheffield has ramped up his political contributions to Republican campaigns. == Collusion allegations ==