In 1510, Dr.John Colet, dean of St.Paul's church, in London, appointed him the first high master of St.
Paul's School, then recently founded by this gentleman's munificence.

In this situation, Lily appears to have taught with great credit to himself till 1522, when he died of the plague, at the age of 56.

For the use of this school, he wrote and published certain parts of the grammar which has since borne his name.

Of the authorship of this work many curious particulars are stated in the preface by John Ward, which may be seen in the edition of 1793.

Lily had able rivals, as well as learned coadjutors and friends.