He joined the Pentagon in 2017 as deputy assistant secretary for industrial policy from McKinsey & Company, a management consulting firm where he specialized in the aerospace industry.

Then-acting Defense Secretary Pat Shanahan tapped Chewning as his chief of staff in January 2019 after the departure of Defense Secretary Jim Mattis. Chewning continued to serve in that role when then-Navy Secretary Richard Spencer stepped in temporarily as acting Pentagon chief while Esper, who was serving as Army secretary, awaited confirmation from the Senate. Chewning then stayed on when Esper took over in July.