Sinclair, born in Virginia, came from a family with a distinguished naval tradition, both his father and grandfather rose to high rank in the U.S. Navy. He entered the C.S. Navy as a masters mate and was promoted to lieutenant in 1863 by Semmes. He served on two famous Confederate ships, the CSS Merrimac and the CSS Alabama. After the Civil War he wrote a 350-page book about his war experiences, Two Years on the Alabama, which was published in 1895 and 1896. He died in 1925 in Baltimore, Maryland.