Lizzy's representative confirmed that Tom did ask the star to be his wife back in May in New York City, reported E! online.
Lizzy was spotted with a diamond ring on her left hand. She wrapped her arm around Tom while the couple stepped out for shopping in Los Angeles.
The 34-year-old actress actually debuted her new sparkler at "Now You See Me 2" premiere in early June.
It will be a busy fall for the engaged couple, who first met in January 2015 while Lizzy was filming in London. Lizzy is expected to return for the fourth season of "Masters of Sex", while Tom is set to star on Amazon's "The Collection".