Agnes Ruth Sengstacken, née Lockhart, was born in a log cabin in the remote southern Oregon Territory, the daughter of Freeman and Esther Lockhart, pioneers who had gone west by wagon train from New York. Her mother was the first local schoolteacher, and her father became the first school superintendent. Agnes's older sister Lillias married Henry Sengstacken, a German-born businessman; after her death in childbirth in 1890, Agnes and Henry married the following year.
She became a poet and writer and an accomplished public speaker. In 1909, she published A Legend of the Coos and in 1942, at age 83, she published Destination West! the story of her mother's life. She also was known as an expert on local shells and fossils, as well as Native American baskets and clothing.
She helped found the Marshfield Public Library in 1910 and served on its board for 27 years.