Haywood began her acting career in 1715 at the Smock Alley Theatre in Dublin. Public records for this year list her as "Mrs. Haywood," appearing in Thomas Shadwell's Shakespeare adaptation, Timon of Athens; or, The Man-Hater. By 1717, she had moved to Lincoln's Inn Fields, where she worked for John Rich. Rich had her rewrite a play called The Fair Captive. The play only ran for three nights (to the author's benefit), but Rich added a fourth night as a benefit for the second author, Haywood. In 1723, her first play, A Wife to be Lett, was staged.
During the second half of the 1720s, Haywood continued acting, and she moved over to the Haymarket Theatre to join with Henry Fielding in the opposition plays of the 1730s. In 1729, she wrote Frederick, Duke of Brunswick-Lunenburgh to honour the future George II of the United Kingdom. George II, as Prince of Wales, was a locus for Tory opposition to the ministry of Robert Walpole. As he had made it clear that he did not favour his father's policies or ministry, praise for him was demurral from the present king. Others, such as James Thomson and Henry Brooke, were also writing such "patriotic" (which is to say in support of the Patriot Whigs) plays at the time, and Henry Carey was soon to satirise the failed promise of George II.
Haywood's greatest success at Haymarket came with The Opera of Operas, an operatic adaptation of Fielding's Tragedy of Tragedies (with music by J. F. Lampe and Thomas Arne) in 1733. However, it was an adaptation with a distinct difference. Caroline of Ansbach had affected a reconciliation between George I and George II, and this meant an endorsement by George II of the Whig ministry. Haywood's adaptation contains a reconciliation scene, replete with symbols from Caroline's own grotto. This was an enunciation of a change by Haywood herself away from any Tory, or anti-Walpolean, causes that she had supported previously, and it did not go unnoticed by her contemporaries.
In 1735, she wrote a one-volume Companion to the Theatre. This book contains plot summaries of contemporary plays, literary criticism, and dramaturgical observations. In 1747 she added a second volume.
After the Licensing Act of 1737, the playhouse was shut against adventurous new plays.