Sidney Brooks DeKay was born on March 7, 1845, probably in New York, to Janet DeKay. His father probably died sometime before 1850. The family lived in Oyster Bay, New York. He applied for admission to the United States Military Academy in September 1861, but it is unclear if he attended.
He served in the 71st New York Militia Infantry, and the regiment defended Harrisburg, Pennsylvania, during the Gettysburg campaign. In May 1864, he received a commission as a 1st lieutenant in Company B of the 8th Connecticut Infantry. The regiment took part in the siege of Petersburg, and he mustered out on December 12, 1865.
DeKay returned to New York after the war, and he earned a living as a lawyer. He served as Assistant United States District Attorney in the early 1870s, and he became military secretary to Governor John Dix in 1873. He married Mina Craven in 1871, and they had at least two children: Eckford, born around 1873; and Janet, born around 1879. He died on August 30, 1890.