Jelf entered the
Royal Engineers in 1865. He was promoted to captain in 1878, to Major and Lieutenant-Colonel in 1885, and Colonel in 1889. He saw service on the
Bechuanaland Expedition (1884-1885), as Director of Military Telegraphs (and was honourably mentioned, Brevet Lieutenant-Colonel). He was later chairman of Sanitary Commissioners,
Gibraltar (1893-1897; for which he was invested as a
Companion of St Michael and St George). He commanded the Royal Engineers, Eastern District, from 1897 to 1901. He retired in 1901 but was recalled to employment as temporary Major-General in the same year, becoming Governor and Commandant of the Royal Military Academy, Woolwich, a post he retained until 1912. While Commandant, he commissioned the academy's chapel in 1902, an act for which he is commemorated by a brass plaque in the chapel. He was also a spokesman for the Church of England Soldiers' Institutes Association, and wrote a 1910 biography of his friend Joseph James Curling, a soldier and priest who had also joined the Royal Engineers in 1865. In 1882, Jelf commissioned a bell for St Mary's church in
Newfoundland's
Bay of Islands, where Curling was priest. ==Family==