The Dollface actress, 35, and her longtime partner Matte Babel have welcomed their second child together,
a girl, she confirmed to PEOPLE on Saturday at the BÉIS Motel pop-up at The Grove in Los Angeles.
Noting that Atlas "loves being a big sister," Mitchell says, "She's so excited. She's been amazing. She helps me change diapers and it's good, she loves it."
Mitchell first announced that she was expecting her second baby in February with an emotional Instagram post.
The You star detailed the difficult experience of celebrating her baby on the road while mourning the loss of her grandmother at the same time.
"Saying goodbye to a loved one and at the same time experiencing the joy of welcoming another into this world is the great cycle of life. It is also my most challenging season to date."
Mitchell tells PEOPLE of her second daughter's moniker: "It's a very special name and I think similar to Atlas, when I heard it, I knew it was just the perfect name." –