Guy was educated at Melbourne University, and took the Scottish triple qualification in 1899, also the M.R.C.P. in 1901. He then served as second resident physician of Monsall Hospital, junior resident surgeon at the Royal Eye Hospital, Manchester, and assistant medical of Brownlow Hill Infirmary, Liverpool. He was in practice at Birkenhead before joining the R.A.M.C. in May 1915. He entered the war in France in July 1915.