The grandson of William Blount, Charles became the most notable of the later holders of the dukedom. The favour which his youthful good looks procured for him from Queen Elizabeth I of England aroused the jealousy of Robert Devereux, 2nd Earl of Essex, and led to a duel between the two courtiers, who later became close friends. Between 1586 and 1598 Blount spent a lot of time on the continent, serving in the Netherlands and in Brittany. He joined Essex and Sir Walter Raleigh in their expedition to the Azores in 1597, along with his brother, Sir Christopher Blount (1565-1601), who was afterwards executed for complicity in Essex's treason. In 1600 Mountjoy went to Ireland as lord deputy in succession to Essex, where he succeeded in suppressing the rebellion of Hugh O'Neill, Earl of Tyrone, whom Essex had failed to subdue.
In July 1601 Mountjoy made himself master of Lough Foyle, and in the following December he defeated O'Neill's Spanish auxiliaries at Kinsale, and drove them out of the country. In 1602 the earl of Tyrone made his submission to Mountjoy in Dublin; and after the accession of James I in 1603 Mountjoy continued in office with the more distinguished title of Lord-Lieutenant (1603 - 1606).
Returning to England, Lord Mountjoy served as one of Sir Walter Raleigh's judges in 1603; and in the same year James made him master of the ordnance and created him Earl of Devonshire, also granting him extensive estates.
About 1590 Mountjoy took as his mistress Penelope, wife of Lord Rich and sister of Essex. After the execution of her brother in 1601, Lady Rich divorced her husband in the ecclesiastical courts. Mountjoy, by whom she had already had several children, married her in 1605 in a ceremony conducted by his chaplain, William Laud, afterwards Archbishop of Canterbury. As he left no legitimate children the Earl's titles became extinct at his death.
{| border="2" align="center"
|-
|width="30%" align="center"|Preceded by:
New Creation
|width="40%" align="center"|Earl of Devonshire
|width="30%" align="center" rowspan="2"|Followed by:
Extinct
|-
|width="30%" align="center"|Preceded by:
William Blount
|width="40%" align="center"|Baron Mountjoy
|}
Reference