He was the son of Henry Lascelles Jenner, who was one of two curates to the Rector of St. Columb Major, and later consecrated though not enthroned as the first Bishop of Dunedin and the grandson of Herbert Jenner-Fust.

[2] His earliest interest in the Cornish language is mentioned in an article by Robert Morton Nance entitled "Cornish Beginnings",[3] When Jenner was a small boy at St. Columb, his birthplace, he heard at the table some talk between his father and a guest that made him prick up his ears, and no doubt brought sparkles to his eyes which anyone who told him something will remember.

In 1876, hosted by the British Archaeological Association, a Cornwall Congress was held at Bodmin, at which Jenner presented a paper on "The history and literature of the ancient Cornish language" based on his findings from the previous year.

[5] In 1877, he discovered, whilst working in the British Museum, forty two lines of a medieval play written in Cornish around the year 1450, known as the Charter Fragment.

He decided to promote an interest in Cornish outside academia, among the people of Cornwall themselves and also organised a special commemoration service of Dolly Pentreath and the centenary of her death.

[5] In 1901, the UK was undergoing a Celtic revival and Jenner formed part of a group (led by L. C. R. Duncombe-Jewell) to establish the Cowethas Kelto-Kernuak (CKK) following the model of the Welsh Gorsedd.

He was made a bard of Goursez Vreiz, the Breton Gorsedh, in September 1903 under the name Gwaz Mikael.

Jenner gave a speech in Cornish on why Cornwall should be duly recognised as a Celtic nation, with a majority of delegates voting to support its admission.

It contained grammar as well as a history of the language and was prefaced by his poem Dho'm Gwreg Gernuak (To My Cornish Wife).

He also translated John 5:1-14 in 1917, which appears in the Cornish language on the entrance walls of Jerusalem's Pool of Bethesda and is headed: Awell san Jowan, an pempes cabydul, gwersy un dhe beswarthek treylys yn Kernewek gans Henry Jenner.

1928 saw Jenner made a bard of the Welsh Gorsedd under the Cornish translation, Gwas Myghal, of his Breton bardic name.

At this time, Jenner called for Cornish to become and optional subject in schools across Cornwall, to little reaction from the authorities of education.

At a time when many people thought the Cornish language had died Jenner observed,[7] The reason why a Cornishman should learn Cornish, the outward and audible sign of his separate nationality, is sentimental, and not in the least practical, and if everything sentimental were banished from it, the world would not be as pleasant a place as it is.He contributed articles on Catholic liturgical rites to the Catholic Encyclopedia.

He and his wife supported the Order of the White Rose, a society of Stuart sympathizers which he had founded in 1891,[14] and of which he was chancellor.