Her brother-in-law, William Pitt the Younger, became prime minister later the same year, and Mary's father, a political ally, became Home Secretary in 1784.
[2] In 1800, the countess's eldest brother, John, an MP since 1786, inherited the viscountcy on their father's death.
Although by nature shy, and often in poor health, the countess was obliged to campaign on behalf of her husband and father and the Pitt government.
Soon after her marriage, she had begun suffering from rheumatism, which confined her to bed for a long period and later often forced her to use a wheelchair.
[2] She died, possibly of liver failure, at the couple's London home in Hill Street, Mayfair.