Between c. 1368 and 1378 he conducted diplomatic missions to Europe, including Italy, which he first visited c. 1373; Italian culture was to become a strong influence on his poetry. During this period he was also connected with John of Gaunt, either receiving his patronage or being in his service, or both. In 1374 he received a house in Aldgate where he lived until 1385 or 1386. Also in 1374 he undertook a series of professional and official appointments including a post as a customs official. His appointments increased in importance and he became prosperous, but in 1385 he left the Custom House and retired to Kent where he was a justice of the peace and, in 1386, knight of the shire and MP. Whether his retirement was voluntary is unrecorded, but he was not re-elected to Parliament after his only session in 1386. Only after Richard II reached the age of majority and took over from Gloucester in 1389 did Chaucer receive any new preferments. In that year he was appointed Clerk of the King's Works and supervised construction, maintenance and renovation work, travelling constantly for two years. He left the post in 1391 and became deputy forester of the royal forest of North Petherton, Somerset. This was his last regular office; his appointment was renewed in 1398 but it is not known for how long he maintained it. Late in 1399 he moved to Westminster, dying there the following year; his tomb in Westminster Abbey became the nucleus of Poets' Corner.

Generally considered to be the greatest English poet of the Middle Ages, Chaucer was recognized during his lifetime and remained extremely influential throughout the 15th century. The canon of his writings has now been established, with only a few short pieces and the translation The Romaunt of the Rose remaining uncertain, but from the 15th to the 19th centuries much spurious material was attributed to him (see chaucerian apocrypha). The chronology of his poetry is less certain, and of the longer pieces only The Book of the Duchess can be attached to a definite event, the death of Blanche, Duchess of Lancaster in 1368. The other works can be put into an approximate order by their relation to each other, the dates of known sources and the influence of French and Italian literary traditions. The Book of the Duchess, some of the short poems and The ABC were written before 1372 and show the influence of French poets. Italian influence begins to appear in the works ascribed to the period 1372-80: The House of Fame, Anelida and Arcite, early versions of The Second Nun's Tale and The Monk's Tale, and some of the lyrics form this group. Between 1380 and 1386 The Parlement of Foules,Boece,Troilus and Criseyde, an early version of The Knight's Tale, The Legend of Good Women and some short poems were written, in all of which the Italian influence appears fully assimilated. The General Prologue and the early stories of The Canterbury Tales were written between 1387 and 1392, and A Treatise on the Astrolabe in 1391-2. The later Canterbury Tales and final short poems date from 1393-1400. The doubtful translation of Le Roman de la rose cannot be placed with certainty; it may belong with The Book of the Duchess in the period of French influence, but the Prologue to The Legend of Good Women links it with Troilus and Criseyde (c. 1385).

Although frequently imitated, Chaucer's blend of humour, realism, philosophical depth, poetic virtuosity, and masterful control of dialogue and character was never matched. The pervasive humour (sometimes vulgar) is directed at various targets, and the poet appears as the butt of his own jokes throughout the canon; in The Canterbury Tales it is Chaucer who cannot tell a tale competently and in the dream-visions he appears as naive, ignorant and foolish. His translations are highly workmanlike and his use of philosophical ideas, though sometimes derived from intermediate texts, consistently demonstrates his understanding of them. As a storyteller he is supreme, and it is for this that he is known best. In the framework of The Canterbury Tales he develops both character and dialogue, and the extended characterization in Troilus and Criseyde has made critics liken the poem to a novel. His favourite verse forms are the decasyllabic couplet and rhyme royal, but The Romaunt of the Rose is in octosyllabic couplets; he also wrote in prose, though less well, in Boece and A Treatise on the Astrolabe, as well as The Tale of Melibee and The Parson's Tale.

Chaucer made a crucial contribution to English literature in using English at a time when much court poetry was still written in Anglo-Norman or Latin. His confidence in the language encouraged his followers and imitators also to write in English and speeded the transition from French as the language of literature.