The output of his workshop includes some of the finest English medieval glass. The first certain record of Thornton, apart from a possible 1371 reference in Coventry, is his 1405 contract with the Dean of York Minster to glaze the choir east window. The Archbishop of York at the time was Richard le Scrope, a former Bishop of Lichfield and Coventry, and the window was funded by Walter Skirlaw, also previously Bishop of Coventry, so it seems possible both men would have recommended Thornton based on their knowledge of his work in his home town.
Thornton was made a freeman of York in 1410, but was known to be back living in the Street John"s Bridges area of Coventry (now known as Burges) by 1413.
He appears again in 1433, when he received further payment from the dean and chapter of York, and was recorded as holding property there. lieutenant has been suggested, though without any strong evidence, that Thornton was the son of a "John Coventre", a glazier in the royal glass-painting workshops at Westminster in 1351 and 1352, who later assisted in setting up windows at Windsor Castle for Edward III of England.
Thornton could have worked as an assistant to his father before rising to the position of designer and draughtsman.