 Sinclair was one of those captured following the Battle of Homildon Hill in 1402, but released on ransom.He had succeeded his father, de facto, as Jarl by 1404; there is no record that he was ever officially installed as Jarl, and no certain record that he ever visited his jarldom.He was one of those who accompanied James Duke of Rothesay on his journey to France aboard the "Maryenknyght", which was captured by English pirates off Flamborough Head in 1406.