He was born in the Gorbals in Glasgow, son of James Sellars, house factor and Elizabeth McDonald.
He was articled to Hugh Barclay from the age of 13 and stayed there until he was 21 when he then moved to the employment of James Hamilton.
He was one of the designers commissioned by the Saracen Foundry to work on a set of standard designs for a series of decorative iron works, for example railings, drinking fountains, bandstands, street lamps, pre-fabricated buildings and architectural features.
- Cove Castle (1867)
- Stewart Memorial Fountain, Kelvingrove Park (1872)
- Mitchell Theatre (1873)
- Greek Orthodox Cathedral of St Luke (1877)
- Kelvinside Academy (1877)
- Glasgow Herald Building (1879)
- New Club, Glasgow (1879)
- Fraser's, Buchanan Street (1883)
- various buildings at the International Exhibition of Science, Art and Industry (1888)
- Anderson College of Medicine, 56 Dumbarton Road, Partick (1888–89). Died during construction. Works completed by John Keppie (1862–1945) as a setting-up commission for the newly formed Honeyman and Keppie (Keppie was Sellars' head draughtsman).
- A Perspective View of the Principal Buildings of the Glasgow International Exhibition 1888, by James Sellars, in pen, watercolour and wash; at the Burrell Collection Photo Library
- Biography, at the Gazetteer for Scotland