Daughter of the Rev'd Isaiah Longmore, Wesleyan home missionary and his wife Martha Susan (Lynax), Lydia came to Australia with her family in 1884. She and her aunt stayed in Adelaide while her mother and father worked as bush missionaries. At the age of fifteen she became a pupil teacher at the North Adelaide School.
In 1894 she graduated from the Teacher's Training College and taught at Kadina, then at Hindmarsh, Goodwood and then East Adelaide. In 1906 she and Elsie Claxton were sent to Melbourne to gain the Infant Teachers' Certificate and two years later she became Infant Mistress of the Currie Street Observation and Practice School. In 1910 she ran SA's first infant school for five- to seven-year-olds, at Norwood. In 1915 she studied the Montessori method of education in Sydney, and on her return established a Montessori class in Norwood. In 1917 she was appointed Inspector of Schools, with a special interest in infant schools, and in 1920, as the infant departments developed a greater autonomy, Inspector of Infant Schools. She was the first woman inspector since Mrs Alice Hills in 1900-2. She encouraged the formation of an Infant Mistresses' Club and of the first mothers' clubs. Her concern for the children in outback locations led her in 1917 to set up a Correspondence School for outback, country and sick children. She also established and supervised several playgrounds in Adelaide.
She retired in 1934, and cared for her elderly aunt. In 1957 she was awarded the OBE for her contribution to the education of little children. She died in 1967 at Allambi Home for the Aged, Glengowrie.