Berta Lorentz was born in Leiden, Netherlands, the eldest daughter of the physicist and 1902 Nobel Prize in Physics winner Hendrik Lorentz and Aletta Catharina Kaiser. Berta was the eldest of four children. Her siblings were Johanna Wilhelmina (born 1889), Gerrit (born 1893, died 1894), and Rudolf (born 1895). At that time of her birth, her father was Professor of Theoretical Physics at the University of Leiden. Her mother, Aletta Kaiser, took care of the children and household and also involved herself with charity work and women's suffrage.