John Ernest Lester was born on 27 September 1888 in Carrickfergus, County Antrim, to Robert John Lester and Henrietta Mary Lester (née Ritchie). The Lesters owned a grocery shop on the Ormeau Road, Belfast, and Seán attended the Methodist College in Belfast until the age of 14 when he began working for the Belfast & County Down Railway in Bangor. Due to being colour-blind, he was forced to leave his railway job and began a career in journalism with the unionist North Down Herald newspaper in 1905, where one of his colleagues was Ernest Blythe. In the next few years he would go on to work for several newspapers including the Dublin Evening Mail, the Dublin Daily Express, and the Galway Connaught Tribune. During this period, he developed an interest in Irish nationalism and politics, Lester joined the Gaelic League, and subsequently the Irish Republican Brotherhood in 1908 and Sinn Féin in 1909. It was around this time Lester changed his name to Seán. Lester was with Eóin MacNeill in 1916 when the Rising began and therefore did not take part in the fighting. He began working at the Freeman's Journal that year as chief reporter and then as news editor covering the ensuing significant years which saw the War of Independence, foundation of the Irish Free State, and Civil War.
Lester's friend and previous colleague, Ernest Blythe, who was by now Minister for Finance, suggested that Lester should join the Department of External Affairs, and thus in 1923, Lester took Blythe up on his suggestion and began working as Director of Publicity. He quickly rose up the ranks to the third highest position in the department, and in 1929, was chosen to act as Ireland's Permanent Delegate to the League of Nations in Geneva, Switzerland. Despite his reservations about his age (he took up his post at the age of forty), not being able to speak any languages other than English or Irish, or that he had never been abroad before, Lester quickly established himself as a diplomat of note in Geneva and worked towards getting the Irish Free State elected onto the influential League Council, which he achieved in 1930. His distinguished work for the Council and with various League committees in the following years led to his being seconded from the Irish service to the position of High Commissioner of the League of Nations in Danzig [Gdansk, Poland] in 1933.
As High Commissioner, Lester came under increasing pressure over the next few years to maintain the authority of the League of Nations in the region, as the Nazis gradually gained more and more power. Following an increasing policy of intimidation and undermining of Lester’s authority by the Nazis, which included placing guards around his house and questioning those who visited him, Lester was promoted to Deputy Secretary General of the League of Nations in Geneva and left Danzig in December 1936.
With the outbreak of the Second World War, Lester struggled to keep the League headquarters in Geneva running as its staff and resources were slashed, various sections were moved to Princeton, USA, and Montreal, Canada, and the pro-Nazi Secretary General, Joseph Avenol, resigned and left for Vichy France in August 1940. Lester then assumed the role of Acting Secretary General of the League, and courageously held out in Geneva with a skeleton staff and limited resources until the end of the war. At the final assembly of the League in April 1946, he was awarded the title of Secretary General, retrospective from 1940. His final task as Secretary General was to formally dissolve the League and dispose of its assets.
Lester returned to Ireland and retired initially to County Wicklow and then to Recess, County Galway, where he indulged his passion for fishing and spent the remainder of his days. He had received the Woodrow Wilson award in 1945 for his courageous service to the League of Nations during the war, and received honorary doctorates from Dublin University and the National University of Ireland in 1947.