Rachel Phillips Marchant was the eldest child of Sophia and Isaac Phillips. Her father, the son of Rachel Seixas and Naphtali Phillips, served as president of Shearith Israel and was a founder of Mount Sinai Hospital. When Rachel was nineteen, her mother died and Isaac remarried, having two more children with Miriam Trimble.

In her mid-twenties she married William Marchant, and they had three children who lived to maturity.