In the glamorous shot, Mitchell makes eye contact with the camera as she breastfeeds her one-month old daughter, Atlas Noa.
Alongside the photo, the 32-year-old added the caption: “Breast friends”.
Since posting, the empowering image has garnered more than 2m likes and received thousands of supportive comments from social media users and fellow celebrities.
Mitchell’s PLL co-star Troian Bellisario commented: “Friggin epic”, while pregnant model Ashley Graham said the actor was “mommy goals”.
While the post proved a hit with the majority of Mitchell’s followers, the star was forced to defend herself when one Instagram user criticised her for “not connecting” with her baby.
“I’m a fan of her, but this picture just says [she's] an attention getter,” the user commented.
“She’s not even looking at the baby, she’s not connected with the baby she’s connected with camera.”
Mitchell promptly hit back at the person, writing: “I missed the part in the baby books that stated I had to maintain eye contact with my daughter while she feeds rather than capturing an amazing moment we were having.
“Pls let me know where I can download your parenting manual, I’ll get right on it!”
In June, Mitchell announced the news of her pregnancy with a topless Instagram post which was shot by the same photographer, Joyce Park.
Alongside the photo, in which her stomach was fully visible, Mitchell wrote: “Does this mean I’m allowed to drive in the car pool lane at all times now?”
The actor also uploaded a two-minute YouTube video entitled “Guess Who’s Preggers”, in which she shared snapshots of her experience thus far and teased the beginning of a new YouTube series called “Almost Ready“ which follows her and boyfriend, TV host Matte Babel.