Shortly after graduation, she decided to return to acting full-time, and studied at the Stella Adler Conservatory, New York University’s Stonestreet Studio and at Michael Howard Studios. Sagemiller was mentioned in Movieline magazine in August 2001. In November 2001, Sagemiller appeared in Gear magazine. In early career, she starred on number of teen films, like Get Over It and Sorority Boys, and was lead actress in 2002 thriller Soul Survivors.

Sagemiller became engaged in July 2006 to her Sleeper Cell co-star Alex Nesic after he proposed to her in the South of France. The couple have two children.

On television, Sagemiller portrayed the character Michelle Ernhardt in the TNT series Raising the Bar from 2008 to 2009. After that series was cancelled, Sagemiller joined Law & Order: Special Victims Unit in a recurring role as ADA Gillian Hardwicke. She replaced Paula Patton when Patton departed the series after just one episode. Sagemiller was featured in ten episodes of the 12th season of SVU. She was replaced in the 13th season by Stephanie March and Diane Neal. In 2013, she was cast as a lead on Chicago Fire spinoff Chicago P.D., but left during pre-production.