Margrett was born in 1623 in Scotland.1 Margrett's father was Hew Forrest and her mother was Janet Watt. Her paternal grandparents were Thomas Forrest and Jeane Weir; her maternal grandparents were Johne Watt and <Unknown>. She had two brothers and a sister, named Alexander, Williame and Jonet. She was the second youngest of the four children.