Legros began etching in 1855; he preferred this medium and produced over 600 plates. Many of his early works are deliberately rough in execution, yet Legros already valued the medium sufficiently to encourage the publisher Alphonse Cadart to establish the Société des Aquafortistes in 1862. Cadart had published Legros's portfolio Esquisses à l'eau-forte (1861), which was dedicated to Baudelaire, a reflection of the poet's important role as mentor and friend following his enthusiastic review of Angelus (ex- Lingard priv. col., Cheltenham) at the Salon of 1859.
In the same year Legros, Whistler and Henri Fantin-Latour, a fellow student at Lecoq's, dubbed themselves the Société des Trois: a union founded more on personal solidarity than on any artistic programme. With Ex-voto (Dijon, Mus. B.-A.), awarded an honourable mention at the Salon of 1860, Legros emerged as a leader of the younger generation of realists, notwithstanding his conspicuous dependence on Courbet. However, this critical success brought no financial security, and in 1863, with Whistler's encouragement, Legros visited London where he found admirers and patrons, notably the Ionides family, and was ardently promoted by the brothers Dante Gabriel Rossetti and William Michael Rossetti. Despite winning medals in the Salons of 1867 and 1868 and receiving the support of the critics Louis-Edmond Duranty and Philippe Burty in Paris, Legros resolved to remain in London. He was naturalized in 1880.
In 1876 Edward John Poynter recommended Legros to succeed him as Professor of Fine Art at the Slade School (see England, §XV). Legros occupied this position until 1893 and introduced etching and modelling to the syllabus, a reflection of his own interests. He was a founder-member of the Society of Painter-Etchers in 1881 and of the Society of Medallists in 1885; the revival of the cast art medal was due almost entirely to his example. Particularly notable are his softly modelled portrait medallions of great Victorians, for instance Alfred, Lord Tennyson (bronze, c. 1882; Manchester, C.A.G.), which reveal his debt to Pisanello.
With its classically inspired economy of form and design, Legros's interpretation of his realist subject-matter exerted a decisive influence in England on the representation of peasant life in the 1880s, comparable with that of Jules Bastien-Lepage. He had a taste for the macabre, which endured from his illustrations of Edgar Allan Poe of 1861 to be absorbed into the Symbolism of the 1890s (as in the series Triumph of Death, begun 1894). In the etchings Death of the Vagabond and Death and the Wood-cutter, these themes coalesce in a stark blend of realism and fantasy which is simultaneously elevated and humane.