Phoebe Sarah Hertha Marks, later known as Hertha Ayrton, was born in Portsmouth, England, the third of eight children to Levi Marks, a Polish watchmaker and clockmaker, and Alice Theresa Moss, a seamstress. Her early life was marked by financial hardship, but she showed early signs of intelligence and independence.

After her father's death in 1861, Hertha was sent to live with her aunt, Marion Hartog, in London. This proved to be a turning point in her life as her aunt ran a school and introduced Hertha to a stimulating intellectual environment. She excelled in her studies, particularly in mathematics and science.