Horniman's 1907 novel, Israel Rank: The Autobiography of a Criminal, was used as the basis of the screenplay of the highly-regarded 1949 black comedy Kind Hearts and Coronets and inspired the 2013 Broadway musical A Gentleman's Guide to Love and Murder.

[10][11] Horniman was described by a contemporary as "a well-to-do bachelor who knew what did and what did not suit him, marriage being in the latter category, the social round in the former".

A reviewer writing in The Athenæum described Horniman's English as "occasionally slipshod", adding: "but he has ingenuity and imagination, and from most unpromising material has produced a readable story".

[14] In January 1903 a three-act play called John Lester, Parson was produced in matinee performances at the Lyric Theatre in London's West End.

His novel Bellamy the Magnificant was published in 1904 by Chatto & Windus, which a reviewer described as "extremely diverting, and an easy popularity may safely be predicted for it".

[5] Horniman was the founder and part-owner with Kate Emil Behnke of the 'Broadlands Nature Cure Sanatorium' at Medstead in Hampshire, the first 'nature-cure' establishment in England.

The plot of the novel, described as "self-consciously decadent", was summarised as "the despised offspring of an aristocratic lady who married beneath her, wreaking his revenge by murdering his way coldbloodedly through his entire family to fortune and a title".

[22][23] A four-act play written by Horniman, The Education of Elizabeth, described as a "blend of domestic melodrama and comedy", was produced in October 1907 at the Apollo Theatre.

[29][4] Horniman's Bellamy the Magnificent, a five-act play described as a "social extravaganza" (and based on his 1904 novel of the same name), was produced by Sir Charles Wyndham at the New Theatre in October 1908.

[35][36] In 1915 Horniman visited a Blue Cross hospital in France treating injured horses, reporting that the majority of cases were "deep and painful saddle cuts", with sabre and bullet wounds in the minority.

[10] The Mystery of John Wake, a drama in three acts written by Horniman and Lechmere Worrall, was produced in April 1916 at the Gaiety Theatre in Hastings, on England's south coast.

[13] Horniman was an investor in the Gattie Transport Scheme and a director of the associated company, the New Transport Co. Ltd.[6] The scheme had been proposed in 1918 by the playwright, engineer and inventor Alfred Warwick Gattie, for "the more expeditious and economical handling of goods" by the railways and motor-lorries, based on a large clearing yard and special machinery to lift and sort the goods.

Following the Jallianwala Bagh massacre at Amritsar in April 1919 he strongly condemned the actions of British officials and released photographs of the incident.

The film starred Billie Burke and was directed by Edward Dillon; it was produced by the Famous Players-Lasky Corporation and released in the United States in January 1921.

[44] In August 1921 the play The Edge O'Beyond, adapted by Horniman and Ruby Miller from a novel by Gertrude Page, opened at the Garrick Theatre in London's West End.

[48][49] Horniman was credited as one of the writers of A Gentleman of Paris, a Paramount Pictures film directed by Harry d'Abbadie d'Arrast and released in October 1927.