Mosher then spent several years accompanying his father on voyages, including an 1866 trip around Cape Horn.
At this point, Mosher left his father's home, holding a series of jobs in Portland, St. Louis, New York City and Philadelphia through the 1870s.
Emerging from the bankruptcy, Mosher opened a wholesale business which manufactured various stationery materials, which finally provided him with a stable income.
In 1891, Mosher published his first book, a poem titled Modern Love by George Meredith, without the author's knowledge or permission.
By the end of Mosher’s publishing program in 1923 there would be 384 titles, 338 reprints of those editions, and 61 “privately printed” books for a total of 783 books grouped into fourteen different series, all limited editions, covering his favorite authors including William Morris, Oscar Wilde, Fiona Macleod (William Sharp), Robert Louis Stevenson, Algernon Charles Swinburne, Walter Pater, Dante Gabriel Rossetti, Richard Jeffries, Vernon Lee (Violet Paget), Edward FitzGerald, Walt Whitman, Andrew Lang, George Meredith, John Addington Symons, Elizabeth Barrett Browning, Robert Browning, Matthew Arnold, Maurice Hewlett, Francis Thompson, Marcel Schwob, J. W. Mackail, Ernest Dowson, John Ruskin, George Gissing, William Butler Yeats, Richard Burton, and others.