Staples was born on 30 June 1849 at 4 Little Charles Street (now St Joans House, Phoenix St.), Somers Town, London.
Her father, Benjamin Staples, was originally a woodcarver, but in 1863 he opened a small dairy shop at 87 Chalton Street, Somers Town.
On 20 April 1869, during her marriage to John James Sainsbury, Mary Ann Staples was working in the dairy shop of a family friend, Tom Haile, at 32 Strutton Ground, Victoria.
Mary Ann continued to work in the shop, but her role diminished as the size of her family grew.
Mary Ann continued to take an interest in the business, however and accompanied John James on inspections of the shops almost until her death, on 9 June 1927, at Bishopsfield, Broadlands Road, Highgate, Middlesex.