Sherwin was born in Upper Dorset Street, in inner-city Dublin in 1905, son of a carter. He left school at 14 and became an apprentive harness-maker. At ten years old, he witnessed intense fighting on North King Street near his home during the Easter Rising of 1916.
As a teenager, Sherwin joined the Fianna Éireann (youth wing of the Irish Republican Army) and participated in the Irish War of Independence. Subsequently, aged 17, he joined the National Army. However, he deserted the army after the attack on the Four Courts that marked the start of the Irish Civil War and re-joined the Fianna as a guerrilla fighter against the Free State.
He was captured after an attack on Wellington Barracks in November 1922 and badly beaten in custody. He eventually suffered a stroke as result and lost the use of his right arm. He was interned at the Curragh Camp and released in 1924
He afterwards ran a dance hall and became chairman of the Fianna.
Sherwin joined Fianna Fáil in 1926, and in 1939 was elected to the party's Ard Comhairle (National Executive). During World War II, he joined the Irish Army as a reserve with other Fianna veterans. He fell out with Fianna Fáil in 1943 after they refused to nominate him as a candidate for a general election. After a brief flirtation with Clann na Poblachta, he first ran as an independent in 1945.
He stood as an independent candidate in the 3-seat Dublin North–Central constituency at the 1957 general election, gaining 15% of the first-preference vote. Colm Gallagher, the Fianna Fáil TD, died in June 1957, just three months after the election, triggering a by-election on 14 November, which Sherwin won.
The constituency was expanded to a 4-seater for the 1961 general election, and Sherwin retained his seat comfortably, winning over 21% of the first-preference vote. However, his vote fell at the 1965 election to less than 6%, and he lost his seat to Labour Party's Michael O'Leary.
Sherwin has been described as "colourful". In 1958, when the Dáil was debating allowing women to join the Garda Síochána, he suggested that "while recruits should not be actually horse faced, they should not be too good looking. They should be just plain women and not targets for marriage".
He remained a republican sympathiser throughout his life and wrote in his memoirs in the 1970s that, "The Provisional IRA can not be expected to disappear as if they had never fought and suffered". On the Northern Ireland question he wrote, "the solution to the northern problem is for the British Army to get out... The Orangemen would not last a week against the northern nationalists supported by southern government. If the Orangemen did fight it would be a good thing in the long run as it would clear the northern area of foreign bigots".
Dublin's Frank Sherwin Bridge, opened in 1982, is named in his honour.
- "Mr. Frank Sherwin". Oireachtas Members Database. Retrieved 12 September 2012.
- Frank Sherwin, Independent and Unrepentant, p5–11.
- Frank Sherwin, Independent and Unrepentant, p16–28.
- Frank Sherwin, Independent and Unrepentant, p34–57.
- "Frank Sherwin". ElectionsIreland.org. Retrieved 12 September 2012.
- "Wanted: Fair cops (but not too fair)". Irish Independent. 16 December 2004.
- Frank Sherwin, Independent and Unrepentant, p204–205.