Tadakatsu was born in Nishio, Mikawa Province as the son of Sakai Tadatoshi, a hereditary retainer of Tokugawa Ieyasu and future "daimyō" of Kawagoe Domain. In 1591, he was awarded a 3000 "koku" fief in Shimōsa Province. In 1600, he was assigned to the train of Tokugawa Hidetada in the Battle of Sekigahara and participated in Hidetada's failed attempt to defeat the Sanada clan at the Siege of Ueda. He was awarded the court rank of Lower 5th, Junior Grade and the courtesy title of "Sanuki-no-kami" in 1607. In 1620, Shōgun Tokugawa Hidetada assigned Tadatoshi to the court of his son, Tokugawa Iemitsu and gained him additional estates with a "kokudaka" of 7,000 "koku" in Fukaya, Musashi Province in 1622. This enabled him to resurrect Fukaya Domain and to style himself as a "daimyō".