Silas C. Swift was born around 1838 in Great Barrington, Massachusetts, to Erastus Swift and Sarah Stevens. His father was a farmer who moved the family to Hartford, New York, around 1846. His mother probably died around 1850, and his father died of bilious pneumonia in Sandisfield, Massachusetts, around November 1859. In 1860, Swift was living in Hartford and working as a farm laborer.
Swift enlisted in the Union army at Fort Edward, New York, on November 6, 1861, and he mustered in as a sergeant in Company E of the 96th New York Infantry on November 19, 1861. According to his enlistment records, he was 5 feet, 7 inches tall, with blue eyes and brown hair. He was shot in the leg and discharged for disability in Albany, New York, on October 16, 1862. He returned to Great Barrington and spent the following year working as a clerk.
On January 25, 1864, he enlisted in the Union army again, mustering in as a private in Company D of the 57th Massachusetts Infantry the following day and collecting a $400 bounty. He reportedly fell ill that spring and was absent from his regiment from May 1864 until the end of the war. The army first sent him to Knight General Hospital in New Haven, Connecticut, but it transferred him to Dale General Hospital in Worcester, Massachusetts, by November 1864. Army officials eventually transferred him to the 117th Company, 2nd Veteran Reserve Corps. He mustered out on October 10, 1865.
After the war, Swift moved in with relatives in Bovina, Wisconsin, and resumed work as a farmer. In 1870, he reported owning $800 of real estate. He moved to Lee, Massachusetts, sometime before 1890, and he married Hattie S. Hewitt there in September 19, 1896. They moved to Kingsbury, New York, by 1900, and in the early 1900s, Swift became an agent for a nursery company. He died in Lee, Massachusetts, on December 28, 1915.