With his friend, George Robertson, Mullen emigrated to Australia, sailing in the SS Great Britain and arriving at Melbourne on 12 November 1852.
[1] Mullen went to the Western District to visit some friends and stayed for six months on a station.
He returned with a brother, William Lowell Mullen, and a good stock of books, and began business at 35 Collins Street East in 1859.
The book-shop was also very successful, a large stock was carried, and it was a centre of intellectual life in the city for a time.
Mullen was married twice, firstly to Eliza Moss (died 15 October 1868) and secondly to Wilhelmina Wild on 17 September 1870.