Lonsdale was born January 28, 1903 in Newbridge, Ireland, a small town south of Dublin. She was the youngest of ten children born to Jessie Cameron Yardley and Harry Frederick Yardley, who was postmaster for the British garrison stationed there. Her father was a heavy drinker, and in 1908, when Kathleen was five years old, her parents were separated. Her mother moved the family moved to Seven Kings, England, a small town east of London. Growing up in England, Kathleen won a scholarship to attend County High School for Girls in Ilford.