In 1870–1871 he visited South America in connection with the hospital accommodation for seamen at Valparaíso, Chile, and other ports; he served on different departmental committees, notably that of 1894–1896 on the Mercantile Marine Fund.

Cosmo's first bias was to poetry, and in 1865 he issued A Dream of Idleness and Other Poems, a collection strongly coloured by his admiration for Wordsworth and Tennyson.

Five years later, a limited edition of the striking ballad of The Christ upon the Hill appeared; it was illustrated with etchings by William Strang.

Besides many contributions to the Academy, Saturday Review, Magazine of Art and other periodicals, he published volumes on The Italian Pre-Raphaelites (1887), The Earlier English Water-Colour Painters (1890 and 1897), In the National Gallery (1895) and British Contemporary Artists (1899).

He wrote a monograph on the illustrator of Alice's Adventures in Wonderland entitled The Life and Work of Sir John Tenniel, R. I.