Thomas Agnew took over sole control of an art dealers founded by Vittore Zanetti (dates unrecorded) in Manchester in 1835. The company had originally been set up in 1817. Agnew renamed it 'Thomas Agnew and Sons'. The firm specialised in old master paintings and drawings made between 1200 and 1850, as well as British paintings and watercolours from the 17th to 20th centuries. A London branch was opened in 1860. It ceased trading on 30 April 2013. Its archives are now held by the National Gallery, London.
The firm was known under several names including Thomas Agnew and Sons, Agnew's and Agnew's Gallery.