He was the son of Mary Boleyn -- the sister of Anne Boleyn and also mistress to King Henry VIII of England. Historians differ as to whether he was the biological child of Henry VIII, or of Boleyn's husband, Sir William Carey, Gentleman of the Privy Chamber and Esquire of the Body to King Henry VIII.
Contemporary rumours stated that Henry was an illegitimate child of Henry VIII. Some 10 years after the child was born, John Hales, vicar of Isleworth, remarked that he had met a "young Master Carey," whom some monks believed to be the king's bastard. The idea that Carey was Henry VIII's secret son has spawned several Tudor legends, even becoming a central part of modern fiction, such as the recent novel The Other Boleyn Girl (based on the life of Mary Boleyn).
Anne Boleyn acted as her nephew's patron and provided him with a top-quality education in a prestigious Cistercian monastery. He was also tutored at some point by French poet Nicholas Bourbon, whose life had been saved from the French Inquisition after Queen Anne's intervention.
Henry's royal aunt was beheaded in May 1536, when he was only ten years old. His mother died seven years later 1543 on her estate in Essex and he was returned to his family.
His Baronial estate consisted of the manors of Hunsdon and Eastwick, Hertfordshire and possessions in Kent. He was also granted an annual pension of £400. On 31 October 1560, Henry was appointed Master of the Queen's Hawks. On 20 April 1561, Henry also became a Knight of the Garter.
Henry seems to have gained some favour with his cousin as she appointed him Captain of the Gentleman Pensioners in 1564; a position making him effectively her personal bodyguard. He seems to have served for four years. On 25 August 1568, Henry was appointed Governor of Berwick-upon-Tweed, Northumberland and Lord Warden of the Eastern March.
The year 1569 was the beginning of the Northern Rebellion (November 1569 - February 1570), a major uprising was instigated by Thomas Howard, 4th Duke of Norfolk, Charles Neville, 6th Earl of Westmorland and Thomas Percy, 7th Earl of Northumberland. The rebellion was expecting the support of Roman Catholic Pope Pius V.
Henry was appointed Lieutenant General of the forces loyal to the Queen. His February victory over Sir Leonard Dacre was instrumental in crushing the rebellion. Nearly three thousand rebels ambushed Henry Carey's party of half that size, but Carey was nonetheless victorious in fending off the assault. A number of the rebels crossed the borders to Scotland but were there targeted by the forces of the Scottish Regent. Henry could still appreciate the courage of Dacre's soldiers, in his letter to the Queen detailing the victory made mention of the rebel charge "the bravest charge that ever I saw!".
The victorious Henry was appointed Warden of the East Marches and represented the Queen in signing a treaty with the Regent on 23 October 1571. On 31 July 1574, Henry became Keeper of Somerset House, the property of the Queen before ascending the throne. He was then named Privy Counsellor in 1577.
16 January 1581 found Henry appointed Captain-General of the forces responsible for the safety of English borders. He was appointed Lord Chamberlain of the Household in July, 1585 and would hold this position until his death.
This did not prevent Elizabeth from appointing him Lord Chamberlain Lieutenant, Principal Captain and Governor of the army "for the defence and surety of our own Royal Person". The appointment occurred on 20 July 1588 in Tilbury.
Henry was a blunt, plain-spoken man with little tact, which often worked to his disadvantage at Court. However, his character and reputation as a successful military leader gained him the respect — and often the affection — of the soldiers who served under him.
Henry also served as Chief Justice in Eyre, south of the River Trent between 1589 and his death. He was Joined Commissioner of the Office Earl Marshal and High Steward of Ipswich and Doncaster. He served as Chief Justice of the Royal Forces between 20 December 1591 and his death. On 2 March 1592, Henry was appointed High Steward of Oxford for life.
Henry Carey died at Somerset House, Strand on 23 July and was buried on 12 August 1596 at Westminster Abbey. On his deathbed his cousin Elizabeth I offered to create him Earl of Wiltshire; however, he refused, saying:
Madam, as you did not count me worthy of this honour in life, then I shall account myself not worthy of it in death.
Two of his sons, George, and John, successively followed him as Baron Hunsdon.
In addition, Henry had several illegitimate children, including Valentine Carey, who eventually served in the military under his father and achieved fairly high rank.