Early years
Wontner was born in Islington, London, as Arthur Wontner Smith. He was the eldest in the family of four sons and one daughter of Thomas Arthur Smith, partner in Wontner, Smith & Co, stocktakers in the City of London, and his wife, Emily Mary, daughter of Thomas Taylor Wontner, stocktaker. He was educated at the North London Collegiate School. His father intended him to follow the same career as his own but his son chose to become an actor. He made his first appearance on the stage at Ryde, Isle of Wight on 18 April 1897, as Sir Thomas Tenby in The Sorrows of Satan. He next joined Sarah Thorne's company at Margate, and during an eight-month stay, played nearly thirty leading parts. He then toured with Louis Calvert as Poins in Henry IV, Part 2, before making his first appearance on the London stage, at the old Globe Theatre on 22 October 1898, as the Comte de Rochefort in The Three Musketeers.
In 1899 he appeared at the Garrick Theatre in Change Alley, at Terry's Theatre in The Upper Hand, and at the Comedy in The Ghetto. He then toured for four years as leading man with Florence West and Edward Compton, adding fifty parts to his already long list. Sir Herbert Tree engaged him to play Baron Bonelli in The Eternal City and other parts in Australia; he opened at Her Majesty's Theatre, Melbourne in September 1903. He remained in Australia until November 1905, playing played fifteen additional parts. On his return to England he appeared at the Comedy in August 1906 succeeding Gerald du Maurier as A. J. Raffles in Raffles.
After a succession of West End roles Wontner made a substantial success when he appeared at the Globe in September 1909, as Raymond Fleuriot in Madame X. In the same year he changed the family name by deed poll, dropping the "Smith" and turning the given name "Wontner" into a surname. The following year he appeared at His Majesty's in Tree's Shakespeare festival, playing Bassanio in The Merchant of Venice and Laertes in Hamlet.
His roles between then and the start of the First World War included Iokanaan, the prophet, in Oscar Wilde's Salome in a 1911 production at the Court Theatre by Harcourt Williams, with Adeline Bourne as Salome, Golaud in Pelléas and Mélisande, the title role in Ben Hur, Edward Voysey in The Voysey Inheritance, Orsino in Twelfth Night and Sir Robert Chiltern in Wilde's An Ideal Husband.