At the age of 12, shortly after she moved with her family to Los Angeles, she was cast as Nicole Bradford in the TV series My Two Dads. The series ran from 1987 to 1990. During the 1990-91 season, she co-starred in the short-lived series Going Places with Alan Ruck, Heather Locklear, and Hallie Todd. The following year, Keanan was cast as Dana Foster in the TV series Step by Step which aired from 1991 to 1998. By 1997 Keanan had changed her name from Staci to Stacy.
Keanan had roles in the short Stolen Poem (2004) and the films Hidden Secrets (2006), and Death and Cremation (2010) with Brad Dourif, Jeremy Sumpter, and Daniel Baldwin. In 2009 Keanan appeared in Holyman Undercover. In 2010, she appeared in a cameo in You Again with Step By Step castmates Christine Lakin and Patrick Duffy. She also co-starred in the film Sarah's Choice.
Keanan received her undergraduate degree from UCLA. She then attended Southwestern Law School and was admitted to the State Bar of California in 2013. She currently practices law under her given name, Anastasia Sagorsky, in the Los Angeles area.
On February 11, 2017, Keanan married Guy Birtwhistle, an actor and Producer.