Grennell was born in
Waitangi,
Chatham Islands, New Zealand, on 11 February 1910, the eldest of five children. Her father, William Henry Grennell, worked as a farmer and fisherman at
Matarakau, Chatham Islands, and was of
Ngāti Mutunga,
Ngāti Tama, Irish and American descent. Grennell's mother, Mary Hazel Teripa Tikao, daughter of
Hōne Taare Tīkao and Martha Hana Toku Horomona, was of
Ngāi Tahu descent. == Education ==