She was born in Huntsville, Alabama and began acting at the age of 8 when she starred as Scout in a local production of To Kill a Mockingbird. She studied operatic singing and ballet professionally, performing at concerts and competitions nationwide. Home-schooled throughout much of her childhood, she began college at the age of 16.

She was a National Merit Scholar (1995) and graduated as valedictorian of her class.  An accomplished violinist, Day was accepted to the Juilliard School of Music but chose to attend the University of Texas at Austin on a full scholarship in violin performance. She double majored in mathematics and music performance, and graduated at the age of 19 in the top 4% of her class.  She is also an avid player of a wide variety of video game genres. Much of her work on The Guild web series was based on her personal experience with video games

However, Felicia is best known for her work in the web video world, behind and in front of the camera. She co-starred in Joss Whedon's Internet musical Dr. Horrible's Sing-Along Blog, which was ranked in the "Top 10 Best TV of 2008" by Time Magazine, Entertainment Weekly and People Magazine and won an Emmy in 2009. She also created and stars in the hit web series The Guild, which is currently in its sixth season.  The Guild has won numerous awards for web video excellence, garnering a PGA nomination for best web series in 2011. She has expanded the brand into numerous merchandising opportunities, including a hit comic book series with Dark Horse Comics.

Her production company Knights of Good produced the innovative web series Dragon Age in conjunction with EA/Bioware in 2011 and in 2012 she launched a funded YouTube channel called Geek & Sundry. Since launching in April 2012, the channel has garnered over one million subscribers and over 200 million views.