Clark Spencer Gilbert was born on January 4, 1840, in Ohio to Hammond and Susan Gilbert. His father was a farmer who owned $600 of real estate in 1850. He grew up and attended school in Jackson, Ohio. By the early 1860s, he was living in Clayton, Wisconsin. He enlisted in the Union army on September 30, 1861, and mustered in as a corporal in Company K of the 11th Wisconsin Infantry later that day. The regiment took part in the siege of Vicksburg. He was promoted to sergeant and then 1st lieutenant, and he mustered out on September 4, 1865.
Gilbert moved to DeKalb County, Illinois, after the war, and he married Mary Jane Wilkinson there on August 27, 1868. They moved to Cedar Rapids, Iowa, soon afterward, and Gilbert earned a living as a carpenter. By 1870, they owned $1,500 of real estate and $350 of personal property. Their son Cliff was born in February 1884, and Mary died soon afterward.
He married Mary W. Forbes on March 30, 1886. They moved to California in the late 1880s, and Gilbert applied for a federal pension in June 1890. By 1900, he was working as a real estate agent in Long Beach, California. He died of “stomach neuralgia” in El Reno, Oklahoma, on May 5, 1901.