He was commissioned into the Royal Field Artillery 17 June 1883; served in South Africa during the Boer War (mentioned in despatches, London Gazette 10 September 1901, Queen's South Africa Medal). During World War I he was mentioned in despatches, awarded the Distinguished Service Order (London Gazette 14 January 1916) and the French Legion of Honour (London Gazette 14 July 1917). He ended the war as a Lieutenant-Colonel in the Royal Tank Corps.
From 27 April 1938 to 22 July 1938 he was a Senator in the Second Seanad, nominated by the Taoiseach. From 7 September 1938 to 14 July 1943 he was a Senator in the Third Seanad, nominated by the Taoiseach. From 8 September 1943 to 5 July 1944 he was a Senator in the Fourth Seanad, again nominated by the Taoiseach.
In 1942 he was involved in the first occasion on which the Irish Senate censored itself. On 18 November 1942 Sir John Keane moved in the Seanad: "That, in the opinion of Seanad Éireann, the Censorship of Publications Board appointed by the Minister for Justice under the Censorship of Publications Act, 1929, has ceased to retain public confidence, and that steps should be taken by the Minister to reconstitute the board." and sparked four days of fierce debate, carrying over to 2, 3, and concluding on 9 December 1942.
He quoted extensively from one book The Tailor and Ansty by Eric Cross, which was banned in Ireland soon after its first publication in that year. The Editor of Debates prudishly excluded the quotation from the Official Report; the entry states only: "The Senator quoted from the book". He taunted William Magennis for thinking that two men embracing in another book amounted to sodomy.
At the end of the debate and much discussion in the public press, his point made, Sir John Keane sought leave to withdraw the motion. The question “That leave be given by the Seanad to withdraw the motion, item No. 2, on the Order Paper” was put and negatived. The question on the main motion was then duly put and declared negatived. However Senators claimed for a division, and the motion was defeated: Tá (for) 2 votes - Sir John Keane and Joseph Johnston - Nil (against) 34 votes.