Geoghegan left Oxford at the end of 1887 and was an instructor of classical languages in London until 1891, when he—along with his widowed mother and siblings—emigrated to the village of Eastsound in the northwestern United States.

Not finding an opportunity to support himself in the fishing/farming economy there, in 1893 he went to Tacoma, in the state of Washington, where he worked as a stenographer for an Anglican bishop, and later in the same capacity for the English and Japanese consulates.

Meanwhile, he unsuccessfully sought a position as professor of Chinese language at the University of Washington in Seattle, and the early days of 1903 he accepted an invitation by the federal judge James Wickersham to come to Alaska as a court stenographer.

Despite the rigorous climate and rough gold mining environment, the informal Alaskan lifestyle and the opportunity to study firsthand Aleut and other native languages of the region appealed to Geoghegan.

His valuable book collection, including many original letters from Zamenhof and other pioneers, as well as other rare artifacts about little known—mainly oriental—languages, were destroyed when the family home in Eastsound burned down in 1906.

Probably Geoghegan's most noteworthy linguistic contribution was the compilation of a dictionary and grammar for the Aleut language of the Alaskan islands, on which he labored from the time of his arrival in Valdez, Alaska, en 1903.