[1][2] Janner was born ברוך אהרון בן יהושע (Baruch Aharon ben Yehoshua) to a Litvak family in Luokė in the Kovno Governorate of the Russian Empire, in what is now Lithuania.

[2] At the age of nine months, his family, who were Orthodox Jews, moved to Barry, Glamorgan, Wales, where his father opened a furniture shop.

[2] Janner was educated at the local school before obtaining a scholarship to attend the University College of South Wales and Monmouthshire at Cardiff.

[2] In 1930, Harry Gosling, the sitting Labour MP for the Whitechapel and St George's constituency in the East End of London died.

[2] 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.