AustLit

Mullen emigrated to Australia from Dublin with his fellow apprentice bookseller, George Robertson (1825-1898). After some months working in the country, Mullen joined him as a assistant in his business as a bookseller and publisher. In 1857 Robertson set up a branch in London. He and Mullen became permanently estranged as a result of Robertson's decision to give the job of managing the London office to his brother, William, rather than Mullen.

Mullen opened up his own successful bookshop in Melbourne in 1859. After his retirement in 1889, his business was acquired by William Robertson who, with A. G. Melville and Leonard Slade, formed Melville, Mullen & Slade, later Melville & Mullen.