He then studied at Johns Hopkins University under the guidance of American geologist William Bullock Clark, where in 1899 he received the first PhD in meteorology ever earned in the United States.

[2] Fassig's official meteorological service began on January 12, 1883, when he entered the Signal Corps of the United States War Department in Washington, D.C.

In May 1883, he was assigned to Fort Myer in Arlington County, Virginia, for instruction, but returned to the Central Office for duty in January 1884.

[6] His contacts with meteorologists from abroad led him to take a year's leave of absence to study in Germany, 1896-7, where he took special courses at the University of Berlin and became a member of the German Meteorological Society.

[8] On leaving San Juan he came once more to the Central Office, this time as Chief of the Climatological Division, which position he retained until his retirement on June 30, 1932.