Situated on the western side of the Lawn, Pavilion IX is a unique structure in that it breaks from the traditional design and introduces the French Neo-classic style, which was popular in the early 18th Century. Jefferson collaborated heavily with Benjamin Henry Latrobe, the first professionally trained architect in the United States, in the design of both Pavilion IX and the Rotunda. Latrobe was also Jefferson's Surveyor of Public Buildings.
Pavilion IX was intended to adapt the Ionic order based on the Temple of Fortuna virilis of Palladio. Jefferson prepared studies for the elevation and floor plans on a piece of engraved graph paper. A notation at the top features the name Latrobe, suggesting the architect's involvement in the design of the facade (which incorporates an entrance niche). Construction was underway by the autumn of 1820. The work was done by individuals also involved in the construction and decoration of the other Pavilions, such as A.H. Brooks (who did the tin roofing) and William J. Coffee (the sculptor charged with decorating the interior, most likely the frieze in the professor's parlor on the second floor).
The first occupant of Pavilion IX was George Tucker, Professor of Ethics and author of the first American science fiction novel, A Voyage to the Moon, which he wrote after taking up residence here in 1825. Another notable resident was Reverend William McGuffy, author of McGuffey's Eclectic Readers. The garden of Pavilion IX once featured an ash tree said to have been planted by McGuffy. The ash tree lived over a century until it died of old age in 1990. A scion was planted in its place.
Numerous small modifications have been made to Pavilion IX since it was built. Recently, in 2007, the University and Mesick-Cohen-Wilson-Baker Architects undertook a detailed study into the building's Chinese-style rails. The findings of this report were used to recreate the original rails from Jefferson's time, which would serve as a prototype for a full restoration of the Lawn's Chinese rails. The 2007 work replaced 1976 wood railings based on historic images, which themselves replaced a 19th-Century, Gothic Revival cast-iron balustrade.