The second son of James Blount, 6th Lord Mountjoy, he succeeded to the family peerage on the death of his elder brother, the 7th Lord, in 1594. After fighting in the Low Countries against Spain, Mountjoy in 1597 accompanied the 2nd Earl of Essex and Sir Walter Raleigh on an expedition to the Azores against Spanish treasure ships. After the defeat of Sir Henry Bagnal's English army in Ireland (August 1598), Mountjoy was expected to assume command there. Essex, who was sent instead, also failed and was subsequently dismissed from office in June 1600, when Mountjoy was appointed lord deputy.

Spanish troops landed at Kinsale in 1601, and the Irish leader, Hugh O'Neill, 2nd earl of Tyrone, marched south from Ulster to join them. Mountjoy waited outside Kinsale, and on Dec. 24, 1601, his cavalry routed the Irish army. The Spanish evacuated Kinsale, and Ulster thus was left open to English invasion. Mountjoy received Tyrone's submission in 1603 and was able to win moderate treatment for him from King James I of England. In the same year Mountjoy was created earl of Devonshire.

He carried on a torrid affair with Lady Penelope Rich, whom he finally married after she obtained an annulment from her husband in 1605.