Horatio Nelson Jackson was born on March 25, 1872 in Toronto, Ontario, Canada to parents Reverend Samuel Nelson and Mary Ann (Parkyn) Jackson. Samuel was a minister who was also born in Canada (Brome), although according to census records Samuel’s father had been born in Massachusetts, thus it is possible that he could claim United States citizenship as well. Mary Ann was Canadian by birth and she and Samuel had seven children, the first two dying in infancy, followed by five sons who all lived to adulthood. Not long after she and Samuel married in 1866, Mary Ann completely lost her hearing and became an expert lip-reader in order to communicate.
According to 1900 census records, Horatio entered the United States in 1873. Whether or not he claimed dual citizenship by virtue of his father perhaps claiming dual citizenship, is not clear. In the 1920’s Horatio applied for passports as a sworn citizen of the United States (although his birthplace is noted in all records as Toronto). Nevertheless, after completing his public school education, Horatio entered the University of Vermont to study medicine at the age of eighteen (his father has also received his degree in medicine from the same institution in 1871).