It holds books and manuscripts with particularly many regarding English literature and history from the 17th-19th century, Oscar Wilde and the fin de siècle, and fine press printing.

It is administered by UCLA's Center for Seventeenth- and Eighteenth-Century Studies, which offers several long- and short-term fellowships for graduate and postdoctoral scholars to use the Library's collections.

After the library’s construction Clark Jr. announced his intent to donate the collection (then around 13,000 books),[3] the buildings and the square-block property to the Southern Branch of the University of California.

His library included the four Shakespeare folios; important editions of Chaucer, Ben Jonson, Byron, Dickens, and Robert Louis Stevenson; works illustrated by George Cruikshank and William Blake; French literature from Pierre de Ronsard to Émile Zola; autograph letters and manuscripts by authors, statesmen, and musicians; and materials relating to the exploration of the American West.

In 2009, nuclear physicist Paul Chrzanowski donated a collection of seventy-two Shakespeare books, published between 1479 and 1731, to the Clark Library.

[10] The Clark Library is one of the most extensive for British literature and history from the English Civil War through the reign of George II (1641–1761).

Among its most valuable collections are the scientific works of Isaac Newton, Robert Boyle, Edmond Halley, John Evelyn, and Sir Kenelm Digby.

The Clark has also taken to collecting books and manuscripts of Wilde's literary circle and the decadent and modernist movements of the 1890s, including the most important editions of William Butler Yeats and many others.

Designed by Robert D. Farquhar, one of California's most eminent romantic architects, its paired cubic reading rooms resemble the Villa Lante, a dual Italian Renaissance composition attributed to Vignola.

In keeping with the collection, its brick and stone facades overlay an English baroque mode similar to that employed by Wren at Hampton Court.

This feature may have been part of the property's appeal for Clark who, after buying it, bought and removed eleven neighboring houses, extended the wall around the entire block, and engaged landscape architect, Ralph D. Cornell to develop plans for a public park.

Willed to UCLA in 1934 with the stipulation that no structure ever rise within one hundred feet of the library, the building stood for the next sixty years in an unfinished landscape gradually emptied by the removal of the house and an observatory.