Kate Douglas Smith Wiggin was born in Philadelphia, the daughter of lawyer Robert N. Smith, and of Welsh descent.Kate herself experienced a happy childhood, even though it was coloured by the American Civil War and her father's death. Kate and her sister Nora were still quite young when their widowed mother moved her little family from Philadelphia to Portland, Maine, then, three years later, upon her remarriage, to the little village of Hollis. There Kate matured in rural surroundings, with her sister and her new baby brother, Philip.
From a literary point of view her childhood was most distinctive for her encounter with the novelist Charles Dickens. Her mother and another relative had gone to hear Dickens read in Portland, but Wiggin, aged 11, was thought to be too young to warrant an expensive ticket. The following day, however, she found herself on the same train as Dickens and engaged him in a lively conversation for the course of the journey, an experience which she later detailed in a short memoir, A Child's Journey with Dickens (1912).
Her education was spotty, consisting of a short stint at a "dame school", some home schooling under the "capable, slightly impatient, somewhat sporadic" instruction of Albion Bradbury (her stepfather), a brief spell at the district school, a year as a boarder at the Gorham Female Seminary, a winter term at Morison Academy in Baltimore, Maryland, and a few months' stay at Abbot Academy in Andover, Massachusetts, where she graduated with the class of 1873. Although rather casual, this was more education than most women received at the time.
In 1873, hoping to ease Albion Bradbury's lung disease, Kate's family moved to Santa Barbara, California, where Kate's stepfather died three years later. A kindergarten training class was opening in Los Angeles under Emma Marwedel (1818-1893), and Kate enrolled. After graduation, in 1878, she headed the first free kindergarten in California, on Silver Street in the slums of San Francisco. The children were "street Arabs of the wildest type", but Kate had a loving personality and dramatic flair. By 1880 she was forming a teacher-training school in conjunction with the Silver Street kindergarten.