Walker produced his first important watercolour, "Strange faces" in 1862 (Yale Center for British Art, New Haven), and in the following year "Philip in Church" which won a medal at the Paris Exhibition of 1867. Walker exhibited at the Royal Watercolour Society from 1864 until the end of his life, becoming an associate member in February 1864. He was later made a full member in 1866, which entitled him to add the postnominal initials 'RWS' after his name. He was also elected an honorary member of the Belgian Watercolour Society in 1871.