Lionel was educated at Queen Elizabeth Grammar School, Carmarthen, and at the Medical School, Cardiff University, where he obtained his B.Sc. degree in 1910. He became a student at Bartholomew's Hospital in 1910, and obtained the diplomas of M.R.C.S., L.R.C.P. in 1912, and graduated M.B., B.S. in 1913. He was then appointed house-physician to Dr James Calvert at St Bartholomew's Hospital. Lionel gained a commission within the R.A.M.C. [T.F.] early in the war and joined the staff of the 1st London General Hospital. In 1916 he went to Mesopotamia serving under the rank of Captain, and joined the staff of the 40th British General Hospital and was afterwards was put in charge of the Pathological Laboratory. He had also, for many months, been in charge of the dysentery section of the hospital. Lionel acquired a mild form of dysentery, which became complicated by pneumonia. He died from this at Basra. He was the son of John and Mary Crossman of Ar-y-bryn, Carmarthen.