In 1855 he exhibited for the first time at the Royal Academy, sending An Ancient Mariner.
He became chiefly known as a successful portrait-painter, and gained frequent commissions for official and presentation portraits.
Among these were portraits of Professor Fawcett, Bishop Colenso (presented by his family to the National Portrait Gallery), Lady Brassey, the Duke and Duchess of Buckingham, and other persons of note.
He also painted some subject pictures, of which Alice in Wonderland, The Challenge, and a few others, were engraved and met with some popularity.
Sidley continued to paint up to the time of his death, which took place at 8 Victoria Road, Kensington, on 9 July 1896.