Janner was born to a Litvak family in Luokė in the Kovno Governorate of the Russian Empire, in what is now Lithuania. He was the son of Joseph and Gertrude Janner. At the age of nine months, his family, who were Orthodox Jews, moved to Barry, Glamorgan, Wales, where his father opened a furniture shop.
Janner was educated at the local school before obtaining a scholarship to attend University College of South Wales and Monmouthshire at Cardiff. He was president of the students' union and edited the college magazine. He graduated with a BA in English and mathematics in 1914, before serving in the Royal Garrison Artillery during the First World War. Having studied law before the war, he was admitted as solicitor in 1919 and established a legal practice in Cardiff.
Janner entered politics in 1921 when he stood unsuccessfully for election to Cardiff City Council as a candidate of the Comrades of the Great War. Three years later he had joined the Liberal Party, but again failed to gain a seat on the council. In 1926 he was elected to the Board of Deputies of British Jews, and subsequently became a member of the executive of the British Zionist Federation.
At the 1929 general election he was the Liberal candidate for Cardiff Central, but failed to be elected. Later in the year he moved to Hendon, north London, and took up employment as company secretary and solicitor for his father-in-law's business.
In 1930, Harry Gosling, the sitting Labour MP for the Whitechapel and St Georges constituency in the East End of London died. Janner was chosen to contest the resulting by-election. The area had a large Jewish population, and he campaigned in opposition to the government's policy on Palestine, but was narrowly defeated. Ten months later at the 1931 general election Janner again contested the Whitechapel seat for the Liberals, this time being returned to the House of Commons. At the next general election in 1935 Janner stood as a Liberal and Anti-Fascist candidate, but was one of many Liberals to lose their seats, with the Labour Party regaining the seat.
Within a year of losing his seat as a Liberal, Janner had joined the Labour Party, and was quickly chosen as prospective candidate for Leicester West, which was held by National Labour with a small majority. In the event there was no election for ten years because of the Second World War.
Janner returned to Parliament ten years later, when he was elected at the 1945 general election as Labour MP for Leicester West, defeating Harold Nicolson the incumbent National Labour MP. When that constituency was abolished for the 1950 election, he was re-elected for the new Leicester North West constituency. He held that seat until he retired from the Commons at the 1970 general election, when his seat was held for Labour by his son Greville.
In 1927, Janner married Elsie Sybil Cohen, daughter of the owner of a furniture store. Their son Greville Janner, later Baron Janner of Braunstone, succeeded his father as the Labour MP for Leicester North-West in 1970.
- "Lord Janner Tireless champion of Jewish causes". The Times. 6 May 1982. p. 14.
- Wasserstein, Bernard (2004). "Janner, Barnett, Baron Janner (1892–1982)". Oxford Dictionary of National Biography. Oxford University Press. Retrieved 21 October 2021.
- "No. 42274". The London Gazette. 10 February 1961. p. 1015.
- "No. 45137". The London Gazette. 26 June 1970. p. 7103.
- Freedland, Michael (21 July 1994). "Obituary: Elsie Janner". The Independent. Retrieved 21 October 2021.
- Elsie Janner, Barnett Janner: A Personal Portrait. London: Robson Books, 1984
- Leigh Rayment's Peerage Pages [self-published source][better source needed]
- Leigh Rayment's Historical List of MPs
- Who Was Who
- Hansard 1803–2005: contributions in Parliament by Barnett Janner