Henry Clay Dennis was born in Cayuga County, New York, in 1844. He was the eldest child of Stephen and Fannie Dennis. His father was a blacksmith who, over time, acquired a modest estate of over $1,200. Henry Dennis was the great-grandson of Reuben Dennis, a Revolutionary War veteran and an early settler of Seneca County. Henry's father Stephen was born in Seneca County in 1815, but found work in Cayuga County, where he married and started his family. Opportunity soon called for him to return to Seneca County, and he moved his family to the town of Ovid in 1847. By 1860, Henry lived with his parents, 3 brothers, and his grandparents John and Sarah, and still attended the public school.
Henry had just turned 18 when he enrolled in the Normal School Company on August 30, 1862 for 3 years or the war. He spent some time with the company, but like many younger soldiers, he was not yet rugged enough to withstand the harsh conditions of camp life. He transferred to the 24th Regiment of the Veteran Reserve Corps on March 15, 1864, and was mustered out in Washington, D.C. on June 9, 1865. He remained in Washington after the war, where he married his wife, Harriett, and had his first child, Naomi.
He moved the family back to Seneca County in time for his son William's birth in 1868, and became a carriage and house painter. William died in 1872, and the Dennis' had two other children, Helen Wortman Dennis and Charles Frederick Dennis. Both died in 1879, only 3 days apart in a diphtheria epidemic, while the family lived in Ithaca, New York, and were buried next to their brother in Sheldrake Cemetery. After that event, all trace of Henry Dennis is lost, although it is known that he died before the publication of Eugene Nash's regimental history in 1911.