Brent was born about April 5, 1652, probably near Aquia Creek in the portion of Northumberland County that became Westmoreland County in 1653 and Stafford County in 1664. His parents were Giles Brent (1600–1672) and Mary Brent, the daughter of a tayac, or emperor, of the Piscataway. A Catholic of both Indian and English heritage, Brent was an anomaly in seventeenth-century Virginia, and like others in his strong-minded family he frequently provoked controversy.

Brent's father had a remarkable career before arriving in Virginia. The younger son of a prominent Gloucestershire family, he had migrated to Maryland in 1638 with three siblings. His Catholicism, affluence, and education led to his quick attainment of positions of authority, such as member of the assembly, councillor, and chief militia officer on Kent Island. More often than not the Brents opposed proprietary prerogatives, both in protection of their own interests and in rallying dissident groups against the Calverts. Despite earlier conflicts Lord Baltimore appointed him acting governor in April 1643. An ardent Royalist, the elder Giles Brent antagonized Protestant supporters of Parliament and helped set off an uprising in the colony before being dismissed from office and transported to England in 1645. After obtaining his freedom he returned to Maryland and was briefly reinstated as a councillor. A final break with the Calverts prompted Brent and his equally influential sister Margaret Brent to move to Virginia about 1649 and settle near Aquia Creek. Giles Brent married the orphaned daughter of a Piscataway leader who had been raised by Margaret Brent and Jesuit missionaries who had converted her and her father to Christianity. If he had hoped that the marriage would secure him a claim to Indian lands and that he could promote her right of succession to her father's title, he was disappointed on both counts. Despite legislation restricting the rights of Catholics and occasional complaints about Catholic influence, the Brent family prospered in Virginia. The senior Brent became a militia officer, and his nephew George Brent (b. ca. 1640) held several responsible public offices in the Northern Neck.

Giles Brent dwelt in two worlds. He learned the Indian language from his mother, but after his father's death early in 1672, he inherited all of his father's extensive landed estate and became a prosperous young planter and a captain in the militia. In 1674 Brent became a local collector of the tobacco export tax. His primary importance in Virginia history arises from his involvement in Bacon's Rebellion. In July 1675, as Captain Brent, he served in a party commanded by George Mason (1629–1686) and George Brent that pursued a contingent of Doeg Indians into Maryland and killed several of them in retaliation for the Indians' having killed some Virginians. Bacon's Rebellion grew from that episode and other clashes on Virginia's frontier. Vague references in some of the surviving records of the struggle have resulted in confusion concerning the roles of Giles Brent and his cousin, George Brent. Giles Brent definitely joined forces loyal to Nathaniel Bacon in order to battle the Pamunkey and other tribes. Referred to during those weeks as Colonel Brent, he collaborated with Bacon until the rebel leader turned his forces against the governor, Sir William Berkeley, in the autumn of 1676 and laid siege to Jamestown. Brent then turned against Bacon and gathered approximately 1,000 men to confront Bacon's forces. When the men learned that Bacon had burned Jamestown, however, they quickly lost heart and deserted Brent, whose role in the conflict then ended.

Brent's last conflicts were domestic in nature. Just as his fiery temperament had led him to confront the neighboring Indians, it began to threaten members of his own household, which consisted of a wife, whose name is unknown, and at least two sons and two daughters. In May 1679 Brent's wife petitioned the governor and Council for protection and a separate maintenance, an action that the prominent Northern Neck attorney William Fitzhugh described as unprecedented in Virginia. The court records are lost, but according to Fitzhugh the petition graphically described Brent's "inhumane usage" of his wife. The Council ordered Brent to live apart from her and "to allow her a Maintenance, according to his Quali[ty] & Estate." A later attorney who examined the court papers before they were destroyed concluded that Brent was "a terrible fellow." Before any further proceedings in the case could take place, Giles Brent died in Middlesex County on September 2, 1679. He may have converted from Catholicism to the Church of England, because his death was recorded in the register of Christ Church Parish and he was buried in the cemetery of that Anglican church in Middlesex County.