Born in Auburn, New York, of Irish parentage, he came from a family that had settled in St Catherines, Ontario, by 1875. Shannon took his first art lessons from a local artist, William E Wright. In 1878 he traveled to England and studied at the National Art Training School under Sir Edward Poynter. As Poynter’s most gifted pupil, he received commissions from Queen Victoria in 1881 and 1882, and although he had intended to return to the United States, the success of his early works persuaded him to remain in London.
In 1886 he married Florence Mary Cartwright, and a year later their daughter Kitty was born. He used them as subjects for his paintings on many occasions, and in these more intimate and informal works he expressed his true artistic talent.
During the mid 1880s, Shannon, together with his fellow American compatriot John Singer Sargent, dominated the field of British portraiture. Shannon’s popularity was encouraged by the patronage of Violet Manners, later Marchioness of Granby, and he became the Manners family’s favorite artist.
Shannon was a prominent exhibitor at the Royal Academy, the Grosvenor Gallery, the New Gallery and the Royal Institute of Oil Painters. He was a founder member of the New English Art Club in 1886, became a Royal Academician in 1909, was President of the Royal society of Portrait Painters from 1910-1923 and was knighted in 1922.
Following his death, memorial exhibitions were held at the Leicester Galleries in London and the Albright Art Gallery and the Cincinnati Museum, USA.