Laffan's first job in journalism was as a reporter for the San Francisco Bulletin, where he quickly became managing editor.
In 1877 Laffan went to work for the New York Sun, then owned by Charles Anderson Dana, as a general writer, often writing art criticism.
[6] He also provided a preface to Stephen W. Bushell’s catalogue of the W. T. Walters collection of Oriental ceramic art.
[9] His funeral service was attended by 500 mourners who travelled to Lawrence on a special train from Long Island City, but by no members of his immediate family.
[10] In 1910, J. P. Morgan donated $100,000 to Yale University to establish the William M. Laffan Professorship of Assyriology and Babylonian Literature in his memory.