Gillis was born in Dauphin, Canada. Gillis grew up in rural Manitoba and got her start on the stage in her hometown playing Grandpa in her first school play. While attending the University of Manitoba, she was discovered by a modelling agency. Shortly after signing with them, she booked a series of commercials for Greenkids starring Lyle Lovett and landed her first film role in the award-winning short film Heart of the World directed by cult film director, Guy Maddin.
Gillis' credits include roles in White Noise 2 starring Nathan Fillion and Shooter starring Mark Wahlberg. Tammy played a leading role in the indie feature film Under the Apple Box, which received an honorable mention at the New Jersey Film Festival winning an Honorable Mention and also screened at the Women’s International Film Festival in Miami in March 2013. She appeared on the Bravo series Girlfriend's Guide to Divorce, as well as the Hallmark romantic comedy A Novel Romance starring alongside Amy Acker and Dylan Bruce and the Lifetime thriller Stolen from the Womb starring alongside Laura Mennell and Larisa Oleynik.
Gillis' television credits include Motive, Less Than Kind, Supernatural, Lost Girl, Blue Mountain State, Endgame, Reaper, The L Word, and Battlestar Galactica.