[1] He previously served as Principal Deputy Assistant Secretary of the Army for Installations, Energy and Environment (ASA IE&E),[2] and earlier as the Acting Assistant Secretary of the Army for Installations, Energy and Environment from October 16, 2017, until January 10, 2019.

He served as a field artillery officer in the U.S. Army at posts including Fort Stewart, Georgia and Ar Ramadi, Iraq, where he was promoted to major.

Gillis was a Director in the Energy Practice at ScottMadden,[7] a general management consulting firm.

His consulting experience focused on working for electric utility clients across North America in mergers and acquisition integration, business plan development, operational assessments, organization design and staffing, and implementation of departmental and corporate strategy.

In February 2020, President Donald Trump announced his intention to nominate Gillis to be the next Assistant Secretary of Defense for Sustainment.