Born in Auckland, New Zealand, the son of a banker, and educated at Queen's College there. He arrived in London in 1901, armed with a bundle of drawings and one editorial introduction. Rountree proceeded to make a unique reputation for himself as an animal artist. His animal drawings, both in colour and black-and white, were sometimes serious, but more often than not were humorous. He was an expert on all aspects of animal, bird and fish life and spent hours at the London Zoo watching his "subjects".

He was a witty, mercurial, bubbling character, "as chirpy", someone once said, "as the sparrows he draws so well". He contributed to numerous magazines and papers, including Punch, The Sketch and The Graphic. He drew many colored covers and inside color plates for the juvenile magazine Little Folks, as well as numerous pictures for children's annuals. He did much of his best work in the coloured comic paper Playtime (1919-1929). Rountree served as a Captain in the Royal Engineers during WWI. He lived for some years in St Ives, Cornwall.