Smith became one of the most popular Evangelical writers of the 19th century. She used her "Christian principles as a protest against specific social evils in her children's books." Her moral tales and semi-religious stories, mainly directed towards the young, were printed in huge numbers.

After her sister submitted, without her knowledge, a story on her behalf ('The Lucky Leg', was a bizarre tale of a widower who proposes to women with wooden legs) Smith became a regular contributor to Household Words and All the Year Round, two popular periodicals begun by Charles Dickens. Dickens would collaborate with many writers to produce his part-work stories. Smith writing under the pseudonym Hesba Stretton (created from the initials of herself and four surviving siblings: Hannah, Elizabeth, Sarah, Benjamin, Anna and the name of a Shropshire village; All Stretton) contributed a well-regarded short story, 'The Ghost in the Cloak-Room', as part of 'The Haunted House'. She would go on to write over 40 novels.

Her break out book was 'Jessica's First Prayer', published in the Sunday at Home journal in 1866 and the following year as a book. By the end of the century it had sold over one and a half million copies. To put that into context; ten times the sales of 'Alice in Wonderland'. The book gave rise to a strand of books about homeless children in Victorian society combining elements of the sensational novel and the religious tract bringing the image of the poor, under-privileged, child into the Victorian social conscious.

A sequel, 'Jessica's Mother', was published in Sunday at Home in 1866 and eventually as a book, some decades later, in 1904. It was translated into fifteen European and Asiatic languages as well as Braille, depicted on coloured slides for magic lantern segments of Bands of Hope programmes, and placed in all Russian schools by order of Tsar Alexander II.

Smith became the chief writer for the Religious Tract Society. Her experience of working with slum children in Manchester in the 1860s gave her books great atmosphere and, of course, a sense of authenticity.

In 1884, Smith was one of the co-founders, together with Lord Shaftesbury and others, of the London Society for the Prevention of Cruelty to Children, which then combined, five years later, with societies in other cities to form the National Society for the Prevention of Cruelty to Children. Smith resigned a decade later in protest at financial mismanagement.

In retirement in Richmond, Surrey, the Smith sisters ran a branch of the Popular Book Club for working-class readers.

Sarah Smith died on October 8th, 1911 at home at Ivycroft on Ham Common. She had survived her sister by eight months.