Bonniwell was born as Hilda Victoria Agnes Whitting in Weston-super-Mare, Somerset, England, on 12 May 1897 to saddler and fruiterer Ben Hutson Whitting and his wife Elizabeth Hares. She had a twin brother, Percy, who lived to be 81, as well as two older siblings, Jessie and Edward, who lived to 85 and 52 years old respectively. Her father died in 1910, when she was 13.

In 1947, aged 50, Bonniwell married her husband, Stanley, for whom she formerly worked as a housekeeper. The couple had no children. She was widowed in 1967.

Bonniwell died in Whitstable, Kent, England, on 11 October 2007 at the age of 110 years, 152 days. Third to Florrie Baldwin and Henry Allingham, she was among the oldest residents of England and the United Kingdom.