Masterson was born on Long Island to Carol Masterson, a manager, and Joe Reaiche, a Lebanese-Australian former rugby player. Her family later relocated to Los Angeles, California.
Her passion for acting came from spending a lot of time on set with her brothers, both who starred in popular television shows, and from seeing the filming process.
- The Young and the Restless
- Definitely Maybe
- Malcolm in the Middle
- Terminator: The Sarah Connor Chronicles
- Grey's Anatomy
- First Day
- Park It Up
- Men at Work
- Masterson is the younger sister of Danny Masterson (Steven Hyde on That '70s Show), Jordan Masterson, Christopher Masterson (Francis on Malcolm in the Middle), and Will Masterson.
- She built a primary school in Nepal.
- Her half-brother, Christopher Masterson, who starred as Francis on Malcolm in the Middle, helped Alanna to score a small role on Malcolm in the Middle.
- She and her fellow cast member Chandler Riggs share the same birthday.
- On July 27 2015, she posted a picture on Instagram, revealing she was pregnant. On November 5 2015, she had the baby, a girl named Marlowe.