Librettist and assistant to Richard D'Oyly Carte

Desprez returned to Britain in 1875. His first piece written for the theatre shortly thereafter was an adaptation of La fille de Madame Angot. When this piece went on the road in 1876, he also wrote a companion piece for it called Happy Hampstead, which was set to music by the theatrical agent and composer Richard D'Oyly Carte. Desprez became a close friend of Carte's and worked with him for many years as Carte's secretary.

At the same time Desprez wrote the texts for ten short works for D'Oyly Carte, most of which accompanied the Gilbert and Sullivan operas on the bills at the Opera Comique and later the Savoy Theatre. These pieces had long runs in tandem with, and sometimes beyond, the runs of the principal pieces, and they were usually played on tour throughout Britain as companion pieces, benefit pieces and short-programme items. Working with composers such as Alfred Cellier and Edward Solomon, Desprez became perhaps the most popular librettist of one-act operas in Britain.

The Nautch Girl, 1891

Desprez's most frequently played work was his 1879 two-act musical play, Tita in Thibet, which was later played in the British provinces by the Majilton company more than a thousand times. It was written as a vehicle for the music hall star Kate Santley. W. H. Seymour, who would become the stage manager of the D'Oyly Carte Opera Company for 20 years, also played in the piece. The story concerns an unusual marriage custom purportedly to be found "in out of the way parts of the world" such as Tibet. The customs of the country permit every wife to have four husbands. The reviewer from The Era did not find the piece "refined" enough for his taste.

Desprez's best work as a lyricist was The Nautch Girl, or, The Rajah of Chutneypore, which played at the Savoy Theatre in 1891–92.

Poet, essayist, journalist, and editor

Desprez' best-known work, however, is a poem, Lasca, about a Mexican girl and her cowboy sweetheart caught in a cattle stampede "in Texas down by the Rio Grande." The ballad-like poem, first published in a London magazine in 1882, has often been reprinted, usually with deletions and changes, and recited in many parts of the English-speaking world. Between 1873 and 1882 at least four other of Desprez's poems had been published, two of which are about Texas.

In 1884, Desprez began writing for The Era, London's foremost theatre paper, and he became its editor in 1893, a position he held until illness forced him to retire in 1913. Desprez also wrote dozens of essays on travel, art, music, and famous personalities that were published in English periodicals, most of them between 1905 and 1914.

Desprez died in London at the age of 63.