William's son, Robert Sempill, was constable of the king's Douglas Castle and was taken prisoner at the Battle of Pinkie Cleugh in 1547. Robert was sometimes called the Great Lord Sempill and supported the Queen Regent, Mary of Guise who was the widow of James V of Scotland. Robert's castle was attacked and seized for his opposition to the Scottish Reformation. He was a faithful supporter of Queen Mary until the death of Lord Darnley and he afterwards joined those who promoted Mary's son as King James VI. He fought against the queen at the Battle of Carberry Hill and was one of the signatories on the warrant for the queen to be held in Loch Leven Castle. He also led the vanguard of the Regent Moray's army at the Battle of Langside in 1568 and was rewarded with a charter to the abbey lands of Paisley. After the Regent was murdered in 1570 Sempill was imprisoned for a year. (See: Robert Sempill, 3rd Lord Sempill).