Mills is the Deputy Director for X-ray Science at Argonne’s synchrotron, the Advanced Photon Source. His main scientific interests are X-ray optics for synchrotron radiation applications and the use of the unique properties of synchrotron radiation, such as the polarization and modulated time structure, for studying condensed matter physics.
After obtaining his Ph.D. at Cornell in 1979, Mills remained at Cornell working as a staff scientist at the Cornell High Energy Synchrotron Source (CHESS).
In 1987, he was awarded a Guggenheim Fellowship to continue this work and held a visiting scientist post at Argonne National Laboratory and at the Synchrotron Radiation Source at Daresbury Laboratory, Daresbury, U.K during that year.
He joined the staff of the Advanced Photon Source in 1988 as the group leader for X-ray optics and beamlines at the APS.