Miriam Cooper was born to Julian James Cooper and Margaret Stewart Cooper in Baltimore, Maryland on November 7, 1891. Her mother was from a devout Catholic family with a long history in Baltimore. Her paternal grandfather had helped discover Navassa Island and made his wealth from selling guano. Her father was attending Loyola University when he met her mother. Her parents had 5 children in 5 years (one died in infancy) including her sister Lenore and her brothers Nelson and Gordon.