The Archives comprise deeds, letters, rolls and other manuscripts dating from the early 12th century in which may be traced not only the history of the Chapel but also that of properties it once possessed in 30 counties in England and Wales. Besides Great Seals of Kings and Queens there are some of the most remarkable medieval seals in the country. Among letters from the famous are those from Henry VIII, Charles II, William Pitt, Lord Howard of Effingham (the Admiral who defeated the Spanish Armada) and William, Lord Burghley (the minister of Elizabeth I).

Illuminated manuscripts include the richly decorated early registers of the Order of the Garter.