Sarah Churchill developed political influence through her long friendship with Princess Anne and her marriage to John Churchill. As a member of Anne's household, she gained unusual personal access, reflected in letters using the familiar names Mrs. Freeman and Mrs. Morley. During the revolution of 1688 she helped Anne leave James II's court and align with the supporters of William of Orange.

Anne's accession in 1702 brought Sarah major household offices, including mistress of the robes, groom of the stole, and keeper of the privy purse. These posts gave her responsibility for royal spending and appointments, and opportunities to advance political interests. She supported her husband's military career and the ministry associated with him and Sidney Godolphin, increasingly pressing the queen to favor the Whigs during the War of the Spanish Succession.

The friendship deteriorated over differences of party, religion, and personal treatment. Anne resented Sarah's pressure and found a less confrontational companion in Abigail Masham. Sarah lost her offices in 1711, and the duke's dismissal followed later that year. Reducing the rupture to a private rivalry alone misses the wider political struggle over ministers and the conduct of the war.

Sarah also managed estates, investments, and the construction of Blenheim Palace, granted to the family after Marlborough's victory in 1704. She argued over costs and design with the architect John Vanbrugh, and continued supervising the project after her husband's death in 1722. Her command of family finances made her a wealthy and forceful landowner in her own right. In later life she cultivated political allies and defended her account of the reign, publishing An Account of the Conduct of the Dowager Duchess of Marlborough in 1742. The memoir is evidence and an attempt to vindicate her own behavior. She died in 1744 after a career joining court office, party advocacy, estate management, and construction of a public reputation.