He attempted to trade in the Far East, but was not successful. In 1835, his father died and left him 30,000 poundss, which he used as capital to purchase a ship, the Royalist. After setting sail for Borneo in 1838, he arrived at Kuching on August 15, 1838, and assisted in resolving an uprising against the Sultan of Brunei.
On September 24, 1841, he became governor of Sarawak. On August 18, 1842, he was declared "Rajah" (prince, or king) of Sarawak by the Sultan of Brunei. He ruled Sarawak until 1868, when he died after suffering three strokes over a period of ten years. During his reign, he expanded the territory he ruled, reduced the practice of head-hunting and piracy in his realm, and increased trade.
He was succeeded as Rajah by his nephew, Charles Johnson Brooke.
A fictionalised account of James Brooke's exploits in Sarawak is given in C. S. Godshalk's novel Kalimantaan.