He drew in a heavily cross-hatched pen-and-ink style, with humans and animals depicted with greatly exaggerated features that are nevertheless firmly rooted in his understanding of correct anatomy.

The following year, he surfaced in the leading humor publication Puck, and his work was also in other periodicals, including Harper's Weekly and Texas Siftings.

He studied with the Philadelphia painter Edward Moran, and he became an apprentice to illustrator E. B. Bensell, noted for his pen-and-ink drawings and wood engravings.

Sullivant was a pen-and-ink artist, working during a time when penwork with meticulously cross-hatched shading, like that of Charles Dana Gibson, was particularly flourishing.

[2] A striking feature was the greatly enlarged heads on hs figures, which, throughout the 1890s, became more and more exaggerated, an innovation pioneered by Frost that was widely imitated by his peers.

[5] Sullivant was noted largely for his animal caricatures and his character types—ethnic types like Irishmen, Jews and Negroes familiar in the American melting pot, as well as farmers, tramps and the suburban families which were emerging at the time.

His "grotesque yet believable"[6] animals were remarkably detailed and anatomically accurate, despite their gross exaggeration, and expressive in the moods, emotions and attitudes they displayed.

He was especially influential in the Walt Disney Studios after T. Hee brought in a collection of Sullivant clippings to use as inspiration for the "Dance of the Hours" sequence in Fantasia.