Jeffreys had a close research association with Hartree, whom she first met as a student at Cambridge in 1925. Her early research under R.H. Fowler on the polarisability of the atomic core was a problem passed on to her by Hartree. Jeffreys moved to Manchester as Assistant Lecturer in Mathematics in 1928 and their association continued the following year when Hartree succeeded E.A. Milne in the chair in Mathematics there. They met less often after Jeffreys return to Cambridge in 1938 but in 1946 Hartree was appointed Plummer Professor of Mathematical Physics at Cambridge.
Jeffreys wrote her reminiscences of Hartree, 'Douglas Rayner Hartree 1897-1958', for Comments on Atomic and Molecular Physics (CAMP), vol. 20.