Jeffery was educated at Dunedin, New Zealand, and at University College, Cardiff, and University College Hospital, London. After qualifying M.R.C.S., L.R.C.P., D.P.H., he held resident appointments at Addenbrook's Hospital, Cambridge and King Edward VII Hospital, Cardiff. He then settled in practice at Merthyr Tydvil. When war broke out he obtained a commission in the R.A.M.C. at the rank of Lieutenant on 7th October 1914., when he became attached to the 11th Bn. Manchester Regiment. He left for Gallipoli early July 1915 in the Cunard S.S. Ascania, and whilst on board took medical charge of about 1,200 men. He was killed during the landing at Suvla Bay on the night of 6-7 August. An officer wrote: “It was in the landing on the evening of the 6-7 Aug. There was a heavy fire, rifle and shrapnel, and many were wounded before they left the lighter. The remainder had to swim or flounder the few yards to the shore. The doctor was kept busy for a short time with those wounded on board, and then he came ashore like the rest. The last time any of us saw him, he was walking about as if he were just holding ordinary sick parade, and seemingly quite indifferent to the terrible hail of lead. In his endeavor to get the wounded, he seems to have walked right into the Turks, and his end was instantaneous. Need I say how much we missed him, and especially in the dreadful days that followed, and we were without ‘our doctor’,” Jeffery was the youngest son of the late Thomas Jeffery Parker F.R.S., Professor of Biology at Dunedin University, New Zealand, and his wife Charlotte Elizabeth, daughter of John Russell. He was also the grandson of W. Kitchen Parker, F.R.S.