Grossmith was born in London and grew up in St. Pancras and Hampstead, London. His father, George Grossmith (1820—80), was the chief court reporter for The Times and other newspapers at the Bow Street police court and a lecturer and entertainer. His mother was Louisa Emmeline Grossmith née Weedon (d. 1882). His brother, George, became famous as the principal comedian of the Gilbert and Sullivan operas with the D'Oyly Carte Opera Company and was also famous as a comedy-sketch artist at the piano. George's children included George Grossmith, Jr., who became a famous actor, playwright and producer of Edwardian musical comedies and Lawrence Grossmith, another actor.

Grossmith was educated at Massingham House on Haverstock Hill in the district of Hampstead then North London Collegiate in Camden Town and Simpson's School, a local private establishment. Interested in art, Grossmith trained as a painter at the West London School of Art, the Slade and the Royal Academy. His goal was to become a fashionable portrait painter, but, although he had portraits and other pictures hung at the Academy, the Grosvenor Gallery and elsewhere, his career as an artist was not as successful as he had hoped. One surviving work is called Such is Life.

Acting career

He turned to acting in 1885, which he pursued until 1917. Joining Rosina Vokes's theatrical company in 1885, he went on tour in the provinces and in America. He first appeared in London at the Gaiety Theatre in 1887 as Woodcock in Woodcock's Little Game. His earliest notable success was made in A Pantomime Rehearsal, and he gained even more success in 1888 opposite Henry Irving at the Lyceum Theatre in Charles Selby's Robert Macaire in (1888). He also played in The School for Scandal at the Globe Theatre (1889) and portrayed Joseph Lebanon in Arthur Wing Pinero's Cabinet Minister (1890).

Grossmith would go on to appear in plays by playwrights such as Henry Arthur Jones, and Jerome K. Jerome, opposite actors such as Herbert Beerbohm Tree at the Haymarket Theatre and with Mrs. John Wood at the Court Theatre. In 1892, he played in W. S. Gilbert's Rosencrantz and Guildenstern, a parody of Hamlet, at the Royal Court Theatre. He became known for playing comedy character roles, noting, "I am almost invariably cast for cowards, cads and snobs", and he was particularly good at portraying harassed, misunderstood little men as, like his brother George, he was small in stature.

He portrayed Archibald Bennick in Arthur Law's The New Boy (1894), Hamilton Preedy in Mr. Preedy and the Countess (1905), Jimmy Jinks in Baby Mine (1911), the Earl of Tweenwayes in The Amazons, Boney in The Misleading Lady, and the Judge in Stopping the Breach, his last role (1917). Critic B. W. Findon wrote, "Among... the artists who thoroughly understand the requirements of farcical comedy, who know how to treat its humour with breadth, and grapple successfully with its ludicrous situations — is Mr. Weedon Grossmith. He is one of the best — I think I may say the best actor of farce on the stage of to-day."

Grossmith was also the lessee of London's Vaudeville Theatre from 1894 to 1896 and Terry's Theatre until 1917.

Author and playwright

In 1892, Grossmith collaborated with his brother George to expand a series of amusing columns they had written in 1888–89 for Punch. The Diary of a Nobody was published as a novel (Bristol, J. W. Arrowsmith, 1892) and has never been out of print since. The book was a sharp analysis of social insecurity, and Charles Pooter of The Laurels, Brickfield Terrace, Holloway, was immediately recognized as one of the great English comic characters. Grossmith created 33 black and white line drawings for the novel. According to biographer Tony Joseph, "In their precise and careful detail these illustrations... reinforce the text to perfection." The work has itself been the object of dramatization and adaptation, including three times for television: 1964, 1979 and 2007.

Grossmith also published another novel, A Woman with a History, in 1896. He also wrote a number of plays, the most successful of which was The Night of the Party (1901). One of his plays, The Duffer, was about students at the Royal Academy, which was successful and enjoyed a Royal Command Performance. In 1913 he published his autobiography, From Studio to Stage.

In 1895, he married Mary Palfrey (1867-1929), an actress. They had one child, a daughter, Nancy (1896-1921). He died in London at the age of 65.