He was born around 1340, the eldest son of King Robert II by his mistress, Elizabeth Mure, although he became legitimised with the formal marriage of his parents about 1349. (They had previously married in 1336, but that ceremony had been impeached as being uncanonical.)
In 1368 his great-uncle King David II created him Earl of Carrick, and he took some part in the government of the kingdom until about 1387, when a kick from a horse disabled him. Probably in consequence of this accident his brother Robert Stewart, 1st Duke of Albany, and not the crown prince himself, became guardian of the kingdom in 1389; but it was the elder Robert who succeeded to the throne on his father's death in May 1390.
At this time he changed his baptismal name of John - unpopular owing to its connection with John Balliol - for that of Robert, and became crowned at Scone in August 1390 as King Robert III. Although he probably attended several parliaments, the new king was seen only nominally as the ruler of Scotland, the real power remaining in the hands of his brother.
Robert III had married Annabella Drummond around 1365. She was the daughter of Sir John Drummond of Stobhall, and by her, in addition to two sons David (who became Duke of Rothesay), and James, who was to succeed him, he had four daughters (Margaret, Mary, Egidia, and Elizabeth). He also had an illegitimate son, James Stewart of Kilbride.
In 1399, however, owing to the king's "sickness of the body," his elder son, David, Duke of Rothesay, gained appointment as lieutenant of the kingdom; but there followed an English invasion of Scotland, serious differences between Rothesay and his uncle, Robert, Duke of Albany, and finally in March 1402 Rothesay's mysterious death by starvation at Falkland Palace.
Fearing for the safety of his surviving son, James, the king had the boy hidden at Dirleton Castle, with a view to smuggling him from there to France. However, a month later, in 1406, Englishmen captured the young James en route, whereupon King Robert died, probably at Rothesay. The king allegedly died from grief over the capture of James.
Robert asked to be buried under a dunghill with the epitaph: Here lies the worst of Kings and the most miserable of men. He was, in fact, interred at Paisley instead of at Scone, the traditional burial ground of the Scottish kings.