His father, the sugar millionaire William Hood Stewart, moved the family from Philadelphia, Pennsylvania to Paris in 1865, and became a distinguished art collector and an early patron of Marià Fortuny and the Barbizon artists. Julius studied under Eduardo Zamacois as a teenager, under Jean-Léon Gérôme at the École des Beaux-Arts, and later was a pupil of Raymondo de Madrazo.
Stewarts family wealth enabled him to live a lush expatriate life and paint what he pleased, often large-scaled group portraits. The first of these, After the Wedding (Drexel University Art Collection, 1880), showed the artists brother Charles and his bride Mae, daughter of financier Anthony J. Drexel, leaving for their honeymoon. Subsequent group portraits depicted his friends — including actresses, celebrities and aristocrats — often with a self-portrait somewhere in the crowd.