Mills began research in the Cambridge University Chemical Laboratory under the New Zealand chemist Thomas Easterfield.

[6] In 1912 the tragic death of Humphrey Owen Jones left a serious gap in the organic chemical staff at Cambridge.

Mills was appointed to a Demonstratorship, and was shortly afterwards elected to a fellowship and a lectureship in natural science at Jesus College.

During World War I, a Mills-directed laboratory was charged with the mission to determine the structure of the chemical pinacyanol and identify a reliable way to synthesize it.

The chemical had been invented by a German company in 1905 and during the war was being incorporated into photographic plates to improve their sensitivity toward the red end of the visible spectrum.

During this period of his career, Mills continued his work on isomerism of the oximes, and also studied spirocyclic compounds, the stereochemistry of molecules with restricted rotation, and cyanine dyes.