Cassidy was born in Chicago, Illinois in 1982. Her parent's occupation is cattle ranchers while her siblings all went into the performance arts.
Most of Cassidy's early days was spent honing her acting skills. As a child she attended the Latin School of Chicago. It is a private institution that focuses on such theater arts as; chorale and muscials.
Upon graduating high school, cassidy moved to Vermont where she attended Middlebury College. By the time she graduated Magna Cum Laude, she had degress in both Spanish and Theater. Shortly after, she took another excursion out of the country to Vietnam.
When she came back to the US, Cassidy went back into her theater work and starred in the New York stage production of "Cigarettes and Chocolate".
Cassidy eventually found her way yo Los Angelas, California. Here she began to expand beyond stage acting and started to star in both tv and films. Her most well known role thus far is that of Tess Mercer on the show Smallville.
As of 2011, Cassidy continues to act and participate in such charities and projects as "Heal the Bay" and "Virginia Avenue Project".