John B. Sanborn was born on a farm in Epsom, New Hampshire, on December 5, 1826. He was the youngest of five children of Deacon F. and Lucy L. (Sargent) Sanborn. He was educated at the Thetford Academy and the Pembroke Academy. He briefly attended Dartmouth College in 1851-52, but left after only one quarter to join the law office of Asa Fowler in Concord. He passed his bar exam in 1854 and subsequently moved to St. Paul, Minnesota, in December of that year. In partnership with two other men, he established a law firm in January 1855.