After her post-doctoral year at Yale, she returned to the UK in 1967 and took up the post of Departmental Demonstrator in the Department of Zoology, University of Oxford. The Professor of Zoology, J.W.R. Pringle, saw zoology as extending from animal studies to molecular studies, and had been partly responsible for bringing David Phillips to Oxford as Professor of Molecular Biophysics. Johnson was able to combine teaching with independent research and continued her work on lysozyme and new crystal studies on other enzymes. In 1972 she received some crystals of glycogen phosphorylase and this was the beginning of a major chapter in her research career. She began a detailed x-ray crystallographic analysis of the protein, which was eight times larger than lysozyme and much larger than any of the other proteins whose structures had been solved at that time.