Sagemiller was born in Washington, D.C. to a political activist mother and a professional American football player father, who played in the NFL for the New York Giants and the Washington Redskins. She attended Georgetown Day School. Her entry into the performing arts was at age 3 when she began studying tap dance, ballet, jazz dance and modern dance. She made her stage debut in To Kill a Mockingbird when she was 9 years old, and soon after she became a regular on her local stage. At 14, however, acting took a backseat to modeling after Sagemiller was scouted by Eileen Ford in a jewelry shop. She eventually left modeling to take an undergraduate degree in Art History at the University of Virginia.

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. In July 2006, Sagemiller became engaged to Sleeper Cell co-star Alex Nesic after he proposed to her in the South of France.

Sagemiller appeared in Movieline magazine in August 2001. In November 2001, Sagemiller appeared in Gear magazine. Most recently, she portrayed the character Michelle Ernhardt in the TNT series Raising the Bar.