Agnes Bertha Smith, later Agnes Marshall, was born in Walthamstow, Essex, England. Her family background is not widely documented, but it is known that she later adopted Marshall as her professional name.

Although specific details are scarce, it is believed that Agnes received early culinary training, possibly within her family or through apprenticeships. This formative training ignited her passion for cooking and laid the foundation for her future expertise.

Agnes likely underwent formal apprenticeships in pastry and confectionery. This would have provided her with structured training in the essential skills and techniques of the time.