Specializing in estate and business law, Darlington handled the legal affairs of industrialist Pierre S. du Pont, including his purchase of Longwood Gardens in 1906.

She graduated cum laude from Wellesley College in 1886 and accompanied her father to the Capitol in Washington, D.C., where she served as his private secretary.

[1][4] She grew up amid wealth and privilege—the Darlingtons' net worth reached $1 million before the Panic of 1893 cost the family much of its fortune.

She specialized in business and property law and famously handled industrialist Pierre S. du Pont's purchase of Longwood Gardens in 1906.

She was active in the local Republican Party and served as secretary and treasurer of the Fire Creek Colliers Company (1909)[10] and trustee of West Chester State Normal School (1925).

As chair of the society's financial committee, she raised funds to buy and improve the Chester County History Center building in 1938.

[13] Darlington never married and lived quietly with her sister at the family home of Faunbrook, a Victorian mansion in West Chester built in 1860 and purchased by their father in 1867.

[6] Judy and John Cummings bought Faunbrook from the Darlington family in 1982 and turned it into a high-end bed and breakfast and wedding venue.

[7][14] Like her father, Darlington was a Quaker of English descent[15] and used distinctive "thy" and "thou" pronouns in letters to her nephew, Smedley Butler.