Leydon, John (1895–1979), civil servant, was born 17 January 1895, fourth child and third son of James Leydon and Kate Leydon (neé Quinn), farmers, of Arigna, Co. Roscommon. He was educated at St Mel's College, Longford, and became a seminarian at St Patrick's College, Maynooth (1913–14). In his 1913 summer exams Leydon finished top of the class in English, second in French and Greek. Maynooth had a strong impact on Leydon's lifestyle and work ethic. He remained a devout catholic and was involved in lay catholic organisations, most importantly in the Legion of Mary. Leydon was a lifelong friend and former colleague of Frank Duff (qv), founder of the Legion.
Leydon entered the British civil service in 1915, achieving first place in Latin in the UK in his entrance exams. He served in the War Office and the Ministry of Pensions. In the latter he acquired many of the technical skills he was to rely on again in the Department of Finance compensation branch after his transfer to Ireland in 1923. In 1919 he was appointed private secretary to George Chrystal, the permanent secretary at the Ministry of Pensions. This brought him into contact with ‘Andy’ Cope (qv), second secretary to the Ministry of Pensions and director of organisation, who became assistant under-secretary for Ireland (1920–22). Cope became something of a mentor to Leydon, and brought him to the attention of those who were forming the Irish Free State civil service.
In 1923 Leydon transferred into the new Irish civil service and entered the Department of Finance as a higher executive officer (later promoted to assistant principal officer) in the finance division, where he was ultimately in charge of post-truce and civil war compensation claims until his promotion to the grade of principal officer in 1927. In the late 1920s he served as secretary to the Dáil Éireann all-party economic committee and represented the Department of Finance on the tariff commission and the de-rating commission. He also spent a short period at the Electricity Supply Board, establishing the company's accounts system.
By the late 1920s Leydon had been singled out as one of the brightest and most talented young men in the Irish civil service. He was offered the post of secretary of the Department of Industry and Commerce by the outgoing Cosgrave administration in 1932, but he refused the appointment as it might appear embarrassing to the departing administration. The incoming minister for industry and commerce, Seán Lemass (qv), renewed the offer, and on 1 May 1932 Leydon was appointed secretary of the department. Leydon, who held very different economic beliefs to Lemass, explained they might have differences of opinion, but Lemass said he wanted no ‘yes-men’ in his department. John Leydon became a much trusted key adviser to Lemass, and the two men proved to be a formidable partnership. Leydon had an important part in the establishment of semi-state companies such as Aer Lingus and Bord na Mona, though he always discounted his achievements, attributing them to Lemass. Through his contacts in Whitehall, Leydon played a significant background role in the negotiation of the 1938 Anglo–Irish agreements. He was particularly associated with the trade agreement and the prices commission, which would monitor tariffs on Anglo–Irish trade. In 1938, in the context of the worsening situation in Europe, Leydon began (with Lemass's approval) to develop cross-border contacts with the Northern Ireland civil service in Belfast, to examine emergency wartime cooperation. Following an anti-partitionist interview by Éamon de Valera (qv) in the London Evening Standard, the tentative discussions stalled.
With the outbreak of the second world war Leydon followed Lemass to the newly established Department of Supplies, where he served as secretary (1939–45). An impartial system of wartime rationing and the establishment of Irish Shipping (1941) were his greatest achievements at Supplies and under his guidance, the department became something of a training ground for talented younger civil servants.
With the closing of Supplies in 1945 Leydon returned to Industry and Commerce as secretary, retiring on 17 January 1955. From 1955 he continued to advise Lemass in an unofficial capacity until Lemass's retirement in 1966. As secretary of Industry and Commerce Leydon was renowned for his logic, clarity, and independence, sometimes referred to as a cold efficiency. He was also known for his skilful and tough negotiating style, a style seen clearly in his negotiations for coal and other supplies with the British during the war and in his post-war negotiations with the US authorities on transatlantic aviation.
Leydon served on the boards of many semi-state bodies. He was chairman of Aer Rianta (1936–49), Aer Lingus (1937–49), Irish Shipping (1941–9), the Insurance Corporation of Ireland (1955–73), Aerlinte Éireann (1958–61), and Cement Ltd. He was director, and later chairman, of the National Bank. After his retirement Leydon remained active in the Irish public service. He was a central figure in the establishment of the Irish Management Institute (1952) and the Institute of Public Administration (1957), of which he was president 1957–9. In 1969 he facilitated the merger of the National Bank and the Bank of Ireland, and was on the board of the Bank of Ireland from 1969 to 1972. In 1948 the Vatican awarded him the honour of Knight Commander of the Order of St Gregory the Great, and in 1961 he was awarded an honorary doctorate of law from the University of Dublin. John Leydon died 2 August 1979 at Our Lady's Manor, Dalkey, Co. Dublin. He is buried in Deansgrange cemetery.
He married (1927) Anne (‘Nan’) (1900–64), daughter of Michael Layden and Margaret Layden (née Cull), they had one child, Mary.