Silas Cooper was mustered into the 7th New Jersey Field Hospital on September 13th, 1861. According to “Records of Officers & Men of the Civil War Vol. I,” Cooper was decommissioned only 3 months later on November 30. “Disability” was sighted as the reason, though no explanation of that disability is given.

He never re-enlisted and passed away in 1886 at the age of 46. He is buried in Evergreen Cemetery in Morristown, New Jersey.