Eldest son of the 2nd Earl of Essex, who was executed for treason by Queen Elizabeth in 1601. On the succession of King James I to the throne of England in 1604, Devereux was restored to his father's estate. In 1606, the King arranged a marriage between Essex and Frances Howard, daughter of the Earl of Suffolk. Since Essex was only fourteen at the time of his marriage, he was sent abroad until he came of age. Meanwhile, Frances became involved with Sir Robert Carr and rejected Essex when he returned to England. The marriage ended in a humiliating divorce in 1613 on the grounds of Essex's impotence.
From 1620-4, Essex served in Protestant armies in Germany and the Low Countries. He joined Sir Horace Vere's expedition to defend the Palatinate in 1620 and served with Prince Maurice of Nassau from 1621. In 1624, he commanded a regiment in the unsuccessful campaign to relieve the siege of Breda. The following year, Essex was appointed Vice-Admiral in Sir Edward Cecil's expedition against Cádiz, which ended in disaster for the English. But although Essex's military career during the 1620s was undistinguished, he earned the affection and loyalty of the troops who served under him because of his willingness to share the same hardships.
Essex alternated his military service with his political duties in the House of Lords. Easily offended and acutely sensitive to the honour of his family name, Essex became estranged from Court life and was associated with the Parliamentary opposition to King James and his successor King Charles I. Because of his criticism of the Duke of Buckingham after Cádiz, Essex was denied command of an expeditionary force sent to Denmark; he then declined an offer to command a regiment on the expedition to La Rochelle in 1627. Essex refused to pay the forced loans demanded by the King in 1626-7, and he supported the Petition of Right in 1628. After the dissolution of the 1628-9 Parliament, he withdrew into private life at his estates at Chartley in Staffordshire.
In 1630, Essex married Elizabeth Paulet, but six years later this marriage collapsed because of her adultery with Sir Thomas Uvedale. When Elizabeth gave birth to a son in November 1636, many believed Uvedale to be the father. Essex once again became the laughing-stock of the Court. He accepted the child as his own and even forgave the Countess, but when the child died the following month Essex despairingly gave up all hope of a successful marriage and an heir to his title.
In 1639, King Charles' uncompromising religious policy led to the Bishops' Wars between England and Scotland. Essex had more military experience than anyone of similar rank in the aristocracy and was appointed second-in-command to the Earl of Arundel, the Earl-Marshal of England, in the army sent against the Scots. However, at the request of Queen Henrietta Maria, Ess