Mrs Harriet Law was born in Essex in 1831, her birth name being Harriet Teresa Frost. She had a Baptist upbringing and became a Sunday School teacher. When G. J. Holyoake spoke at Philpot Street, Whitechapel, in the East End of London, she tried to argue against him in defence christianity, but in 1855 "saw the light of reason" and became one of his most active supporters.
In 1859 she became a salaried lecturer for the secular movement. Lectures she gave at Hebden Bridge in 1870, on Martin Luther and Tom Paine, were noisily disrupted by local christians and she was punched in the face while making her way back to the hall through a large mob. When she lectured in Woolwich on "How I became a freethinker and why I remain one", an outraged writer in the local press attacked it as "the infidel lecture".
She edited the Secular Chronicle (1870? - 79) in which she included profiles of freethinking women, such as Mary Wollstonecraft. She even considered the biblical Eve to be a freethought heroine! - For encouraging Adam to partake of the tree of knowledge.
In 1866 she stood as candidate for president of the newly formed NSS against Bradlaugh, Robert Cooper and John Watts. However, because of her involvement in socialism and trades unionism she was edged out of influence in the NSS by Bradlaugh.
She was mandated to represent "The Central Section of Working Women" at the 5th Congress of the International Working Men's Association at the Hague 1872. She is listed as the only woman on the General Council 1869 and 1878. Harriet Law's proposition moved at the General Council meeting of August 17, 1869 would have meant the transfer of the Church's property and income to schools.
In 1877, following disagreements with Bradlaugh and Besant, she, Holyoake and Watts left the NSS and set up the British Secular Union which lasted to 1884. Nevertheless, Harriet Law was one of the speakers, alongside Charles Bradlaugh, Annie Besant, G. J. Holyoake and others, at the opening of Leicester's Secular Hall on 6th March 1881.