Mary was born in Calcutta, the daughter of Aruna, an Indian woman, and a British soldier, she grew up in the Calcutta barrack lines. Her father always said she was pretty enough to marry an officer and rise in the world, but no officer with ambition would marry a half-caste.

After her parents died in an epidemic, for her own protection, she chose to marry a kind man, Sergeant Jem Bickerstaff of the 33rd Regiment of Foot. Bickerstaff died of fever after the company was transferred from Madras to Mysore, however, leaving her a widow at twenty-two. Four months later, she had taken Private Richard Sharpe as her protector, and had half convinced herself she was in love with him. It was she who lit the fire of ambition in Sharpe, telling him he could make sergeant himself. He scoffed at the idea, at first, having all ready been busted down from corporal once, but when the opportunity arose, he realized he could indeed be better than he was.

Mary offered to teach him to read, but circumstances left that task to to others. When Sharpe joined William Lawford in an mission behind enemy lines, Mary ran with them, both to give them an interpreter, and to protect her from Sergeant Hakeswill and Captain Morris who colluded to have Sharpe killed so that they could steal Mary and sell her to a pimp.

Once in Serinapatam, she was given a post in the home of Appah Rao; as English tutor to his children. Rao's wife, Lakshmi, stripped Mary of her western clothing, and dressed her in a sari, altering her appearance so much that Sharpe was slow to recognize her when next they met.

Mary fell in love with Rao's bodyguard, Kunwar Singh, and after providing a pistol to Sharpe to aid in his escape from the prisoner cells, she remained behind to marry Singh, taking the name Aruna for herself.