Jonas Benjamin Phillips was the eldest son of Abigail Seixas and Benjamin Jonas Phillips. His father, a lawyer, was the son of Jonas Phillips and Rachel Mendes Machado Phillips. Among his siblings were Dr. Manuel Phillips, Rachel Machado Phillips Levy, Naphtali Phillips, Aaron Phillips and Zalegman Phillips. Jonas’ mother was the daughter of Zipporah Levy and Benjamin Mendes Seixas, and among her sixteen siblings was Moses Benjamin and Esther.
Born and raised in Philadelphia, Jonas would remain in that city for his entire, life. He worked as a merchant and never married.