Professor Mills's current activities are in the field of metrology: the science of measurement. He is President of the Consultative Committee for Units (CCU) at the International Bureau of Weights and Measures at Sèvres (the BIPM, Bureau Internationale des Poids et Mesures). This is an international committee responsible for revising, maintaining, and developing the International System of Units (the SI, Système International d'Unites).

He has published almost 200 papers. These papers deal with various phases of spectroscopy, both experimental and theoretical and beyond.s He has dealt extensively with vibration-rotation molecular bands, considering both their locations and intensities.