Lord of Galloway and leader of the 'Black Douglas' clan. Douglas was the illegitimate son of Sir James Douglas (c.1286 - 1330). He extended the Douglas lands and power, gaining the Barony of Bothwell and its castle through marriage (1362) and the Lordship of Galloway, together with lands in the eastern part of that region, from King David II (1324 - 71) in return for his loyalty (1369). He succeeded to the Earldom of Douglas after his cousin James Douglas (c.1358 - 1388) was killed at the Battle of Otterburn.
He rebuilt Bothwell Castle and built Threave Castle, between 1360 and 1390, and it was in the latter that he died. His body was returned to Bothwell, where he was buried in the Collegiate Church of St. Bride's, which he founded.