Steward Southgate was born on September 8, 1703, in Combs, Suffolk, England, the son of Richard Southgate and Elizabeth Steward. In 1717, the Southgate family sailed for New England, and in 1718, they were among the first to settle Leicester, Massachusetts. On March 28, 1735, Steward married Elizabeth Scott, with whom he had five children: John (b. 1737), Robert (b. 1741), Margaret (b. 1743), Sarah (b. 1744), and Steward (b. 1748). In 1744 or 1745, Steward left the Church of the First Society in order to join the Quakers. He died in 1765.
John Southgate, Steward's oldest son, lived and worked in Leicester for most of his life. He married Eleanor Seargent, with whom he had several children, including John, Eliza (1795-1839; married Jacob Bigelow), and Sally. He worked as a land surveyor in Massachusetts and served in the Revolutionary War, first as adjutant to a regiment of minutemen, and then as a captain-lieutenant after 1777.
One of John Southgate's brothers, Robert Southgate, moved from Leicester to Scarborough, Maine, to study medicine. There, he married Mary King (sister of the noted federalist Rufus King) in 1773; they had at least 12 children. Robert amassed a large amount of land in Scarborough, and was eventually appointed Judge of the Court of Common Pleas.
Another brother, Steward Southgate, Jr., also served in the Revolution and resided first in Hardwick, Massachusetts, and then Barnard, Vermont. He married Deborah Raymond in 1773. In 1795, the couple lost five children to scarlet fever within 18 days. Sarah Southgate married Azariah Dickinson and settled with him in Hadley, Massachusetts. They had several children, including Asenath Dickinson.