A writer in The Guardian at the time of his death describes him as "remarkable for an almost feminine gentleness of manner, and for the unaffected simplicity of his life."
Its leading merit consisted in its author's judicious use of the system and researches of recent German scholars — "in applying the method of Ollendorff to the syntax of Buttmann."
J. E. Riddle he published in 1847 an English-Latin Lexicon, based on a German work by Dr. C. E. Georges, which cost him, he wrote in the preface, "many years of labour."
Between 1848 and 1853 he edited, in twenty-five volumes, portions of all the chief Latin and Greek authors, and published handbooks of classical antiquities, the Anticleptic Gradus and similar works.
In his classical work he depended largely on Madvig, Krüger, Zumpt, and other less known scholars; his treatment of modern languages was also based on German models, and Arnold was generally ready to acknowledge his obligations to foreign writers.
Arnold's contributions to theological literature also included five pamphlets on ecclesiastical questions raised by the Oxford Movement; an abridgment of an American version of Hengstenberg's Christology; two volumes of sermons, one published in 1845, and the other posthumously in 1858; and Short Helps to Daily Devotion (1847).
In an article in Fraser's Magazine for February 1853, afterwards published in pamphlet form and attributed to John William Donaldson, an attempt was made to discredit Arnold's classical schoolbooks.