He was born in Cumberland, the eldest of nine children. His father died of natural causes on 30 May 1485 and Thomas succeeded him as Baron Dacre of Gilsland. Dacre took part in the Battle of Bosworth Field (22 August 1485) on the Yorkist side against Henry Tudor, Earl of Richmond, when Richard III of England was defeated and Richard killed. He however quickly made peace with the victor. This early support for the House of Tudor earned him some favor with Henry VII who would continue to trust his services for the remainder of his reign. Henry VII named him a Knight of the Bath in 1503. He swore loyalty to Henry's son and successor, Henry VIII of England in 1509.