Thomson was born on July 5, 1841 at Chambersburg, Franklin County, Pa. He was the son of Alexander and Jane (Graham) Thomson. His grandfather, Alexander Thomson, emigrated from Scotland in 1771 and established himself in the Cumberland Valley. His son, Frank's father, became a lawyer and took a prominent part in the political life of his day. He represented his district in Congress, 1824-26, and in 1828 was chosen judge of the Sixteenth Judicial District of Pennsylvania. He was also professor of jurisprudence in the law school of Marshall College, the school at that time being located at Chambersburg.