Lewyn was the principal master mason to Durham Cathedral and the Palatinate and may have been the John 'Loewyne' who was a working mason on repairs to Westminster Palace in June 1351. As an architect, he was sent from Durham in 1364 to Coldingham Priory and from 1367-74 had charge of building the great kitchens of Durham priory, with its remarkable vault. In 1368, Lewyn was appointed by the crown to repair Bamburgh castle, and probably rebuilt the Durham keep. By this time, Lewyn was no doubt regarded as the principal military architect in the North and was commissioned to build Bolton Castle in Yorkshire. It may well be that Raby, Sheriff Hutton (1382) and perhaps Lumley, are Lewyn's work. In 1386, Lewyn was superintending building works for the crown at Berwick, and may have been concerned with the great new donjon at Warkworth for the earl of Northumberland. In 1392, he undertook to repair all defective roads at Brancepeth and in the 1390s, was working at Finchale.
Lewyn was a distinguished master in a noble and functional style, but in the geometrical vault of the Durham prior's kitchen, he showed an outstanding aesthetic awareness. His career is the most remarkable of any provincial architect of the entire Middle ages.