She was born in Coalisland, County Tyrone, Ireland, on 16 September 1840.
[1] After arriving in New Zealand, Milne was employed as head milliner at Robert Graham's soft goods store in Shortland St.
In 1866, partnering with her sister Charlotte, they took over a haberdashery and draper's store in Wyndham Street, relaunching as M & C Milne a "Millinery and Drapery Establishment".
In 1875 Charlotte married Henry Choyce, and after the birth of their first child Henry replaced Charlotte as Mary Jane's business partner and the business was renamed Milne & Choyce.
[2] In 2012, Milne was posthumously inducted into the New Zealand Business Hall of Fame.