[1][2] Dennis was born into a farming family in Langney in Sussex on 17 September 1869 and grew up in Cuckfield, Guestling and St Leonards.
[3] Dennis was ordained in St Paul's Cathedral in 1893, and served for some months as an assistant curate in the parish of St. Marys, Islington.
He was acting Vice-Principal of Fourah Bay College, Freetown, Sierra Leone (1893–1894)[3] before joining the Niger Mission in November 1894.
He married Matilda (Mattie) Silman in 1897, and in the same year, following the death of H. H. Dobinson, was appointed mission secretary.
[7] He was a key player in the development of Union Igbo, a written standard intended to serve all dialects.
One of their suitcases, containing a draft of the Igbo-English dictionary and part of an Igbo grammar, later washed up on the coast of Wales.