Bell was articled as an architect to his uncle, Samuel Knight.
On his return he shared a studio with George Frampton. With Frampton he created a series of designs for an altarpiece which was exhibited at the Arts and Crafts Exhibition Society and later installed in the Church of St Clare, Liverpool.
From 1895 to 1899 Bell was an instructor at the Liverpool University School of Architecture. During this time he became associated with the Della Robbia Pottery in Birkenhead and was also becoming increasingly successful as a book designer and illustrator.
In 1911 Bell was appointed chief of the design section at the Glasgow School of Art, and from 1918 to 1924 he was professor of design at the Royal College of Art. He continued to paint and exhibited at the Royal Academy, the New English Art Club and the Royal Society of Painters in Water Colours, as well as at the Society of Graphic Art's first exhibition in 1921. He designed the great mosaic in the tympanum at Westminster Cathedral from sketches left by the architect John Francis Bentley; the work was completed in 1916. Bell worked from 1922 on mosaics for the Palace of Westminster. Depictions of Saint Patrick of Ireland and Saint Andrew of Scotland were erected in the Central Lobby; in Saint Stephen's Hall, one panel was erected depicting Saint Stephen, King Stephen and Edward the Confessor and another showing Edward III presenting the design for St Stephen's Chapel to his Master Mason, Michael of Canterbury. The last of these mosaics was unveiled in 1926.