James Sinton Sleator was born in County Armagh and studied at the Belfast College of Art. In 1910 he joined Orpen at the Metropolitan School of Art, Dublin. In 1912 he went on to the Slade and from there to Paris where he spent a brief period before the outbreak of the Great War.

By 1915 he was back in Dublin to teach at the School of Art and to exhibit for the first time at the Royal Hibernian Academy. Referring to his four portraits in this exhibition, The Studio commented '…one recognises "quality" of a very unique kind. His rapidly executed head of a man in a red coat and his self-portraiture full of distinction and beauty of tone.'

Within two years he had been elected Academician. In 1920 Sleator became a founding member of the Society of Dublin Painters although his work only appeared in the inaugural exhibition, and in 1922, he left Ireland to spend five years working in Florence. When he returned he set up a studio in London where he established a successful portrait practice.

As Orpen's studio assistant, Sleator often painted the sitter's clothing and background areas in his master's portraits, and shortly after Orpen's death in 1931, Sleator was asked to complete a number of his unfinished commissions, including portraits of Prince Arthur of Connaught and the Duchess of Westminster. He exhibited at the Royal Academy, was friendly with Lavery and for a time took over his most celebrated pupil, Sir Winston Churchill.

In 1941 he returned to Dublin, and in 1945 was elected President of the Royal Hibernian Academy, a post which he held for three years until his death, when the position was taken by Seán Keating. His obituarist, Estella Solomons, noted his 'natural kindness' and 'abiding humour' and suggested that is was his 'warm and tolerant humanity which gave his work its real distinction'.