One of Cook's earliest professional engagements was in the obscure Michael Balfe opera, Letty the Basketmaker, produced by John Hollingshead at the Gaiety Theatre in London in 1868.
[2] Cook then joined one of Richard D'Oyly Carte's touring companies in 1878 in Gilbert and Sullivan's The Sorcerer, playing the vicar, Doctor Daly,[3] and also Old Matthew in the curtain-raiser Breaking the Spell, by H. B. Farnie, based on Jacques Offenbach's Le violoneux.
[2] Cook left the company upon his return to England, appearing later in 1880 and 1881 in The King's Dragoons in Manchester and Liverpool, and in then in La Belle Normande and The Grand Mogul in London.
In 1883, Cook joined Kate Santley's company at the Royalty Theatre in The Merry Duchess by George R. Sims and Frederic Clay in the role of Farmer Bowman.
In 1884–86, he was back with D'Oyly Carte, touring as Dick Deadeye in Pinafore, the Sergeant in Pirates, Archibald Grosvenor in Patience (in 1884 only), the Earl of Mountararat in Iolanthe (in 1885 only) and Pooh-Bah in The Mikado (in 1885–86).