Garrett was the daughter of Newson Garrett (1812–1893), a prosperous merchant, and Louisa Garrett (née Dunnell; 1813–1903). She was the seventh of eleven children. She attended a boarding school at Blackheath, near London.[1]
She and her cousin Rhoda Garrett were employed by London architect John McKean Brydon in 1871, giving them an entry into training that no other practice was willing to allow, as architecture was not considered suitable for women. The cousins opened the first interior design company in Britain to be run by women. R & A Garrett opened in mid 1875, in a flat behind Baker Street station, moving to 2 Gower Street in Bloomsbury c.1884.[1][3][4]
Agnes's older sister was Elizabeth Garrett Anderson, who was the first British woman to qualify as a doctor. Elizabeth set up a pioneering hospital for women, renamed after her death the Elizabeth Garrett Anderson Hospital, and Agnes contributed to its design. For example, she designed the fireplace for the entrance hall, which is now open to the public as a historical gallery within the refurbished UNISON headquarters building.[5]
She was painted by the artist Annie Swynnerton in 1885. The painting survived and it was identified by the historian Elizabeth Crawford in the 2020s.[6]