reynolds was born on august 20, 1869 to dexter and catherine reynolds (née cuyler) in great barrington, massachusetts. when catherine reynolds died in 1875, dexter placed marcus and his brother cuyler under the care of dexter's sister laura, widow of baynard van rensselaer, moving them to 98 columbia street in albany, new york. the reynolds' family connection to the van rensselaer family allowed the boys to grow up surrounded by "wealthy, socially, and politically connected dutch and new england heritage".