After taking the M.R.C.S., L.R.C.P., Ainslie qualified M.B., B.Ch. in 1912. He took up a temporary commission in the R.A.M.C. on 15th November 1915. He was the son of the Rev Lancelot Parker Booth, D.D., M.D., and Olive Booth of Durban, Natal, South Africa, and the husband of Martha Maden Booth (nee Barlow) of 2 Beechcroft Road, Oxford.