Charles Syphax married another Custis slave, Maria Carter. Both had been Mount Vernon slaves where they had worked as household servants. Maria Carter Syphax was the daughter of Airy Carter, a slave maid of George and Martha Washington and later George Washington Parke Custis. In 1826, Mr. Custis gave Maria Carter Syphax and her children their freedom and a seventeen-acre plot within the Arlington plantation. The Syphaxes had ten children who lived as free persons on the estate. According to Syphax family tradition, George Washington Parke Custis was the father of Maria Carter Syphax.

The descendants of Charles and Maria Syphax, beginning with their children, have held positions of leadership in the business and community life of Arlington County. Their son, John Syphax, who was born free and educated in Washington, D.C. schools, became a property owner in Arlington County. He held several elective offices including supervisor of the Arlington Magisterial District, delegate to the General Assembly, and justice of the peace.

His brother, William Syphax, served as Chief Messenger of the Department of Interior. He was also a leader in the effort to establish public high school education for African Americans in the Washington, D.C. school system. Many Syphax descendants still live in Arlington County.