Forster is known from a number of small portraits in pencil on vellum, and according to Lionel Cust they are "drawn with exquisite care and feeling".
[2] The majority of these were no doubt intended for engraving as frontispieces to books, and the following were so engraved by Michael Vander Gucht and others: John Savage, Sir Thomas Littleton, the Speaker, William Lloyd, Bishop of St. Asaph, Humphry Hody, John Newte, and others.
[2] Unlike David Loggan, Robert White, and John Faber Senior, who drew portraits "ad vivum" in the same style, Forster does not appear to have been an engraver himself.
A number of his drawings were exhibited at the special Exhibition of Portrait Miniatures at the South Kensington Museum in 1865; they included Robert, Lord Lucas, Archbishop Ussher, Sir Thomas Pope Blount, Baronet, Lady Blount, John, Lord Somers, and Admiral Sir George Rooke.
A drawing of Margaret Harcourt was acquired for the print room at the British Museum.