McWilliams was born in Berlin, Germany, but moved to United States, there raised primarily in Kansas City, Kansas. She graduated from the prestigious Theater School of DePaul University in Chicago, and later moved to Los Angeles, California. In 2005, McWilliams landed the recurring role as Kacee Franklin, C-Note's wife, in the Fox drama series, Prison Break. The following year, she appeared in film The Lake House, and later co-starred in the independent films Of Boys and Men and One Small Hitch.