Born in Baltimore, Phineas was the son of immigrant Dr. Jonathan (Jonas) Horwitz and his wife, Deborah Andrews, the granddaughter of the Revolutionary War financier Haym Salomon.

[5] Following the end of the conflict, Dr. Horwitz was posted to the USS Constitution, which was joining the Mediterranean Squadron following its refitting in 1847.

[5] While the Princeton stationed in Philadelphia, Dr. Horwitz met his wife, Caroline Norris,[3] whom he married in 1854; the couple would have seven sons and two daughters.

[6] Caroline herself was a published author, including Swanhilde, Fairy Lure, and Twentieth Century Chronology of the World.

[7] Phineas joined the USS Supply in 1855, and the ship performed duty at stations near Africa and Brazil.

He served there throughout the American Civil War,[5] receiving a promotion to the grade of Surgeon on April 19, 1861; a rank equivalent to lieutenant commander.