In a statement shared to Instagram, the actor, who previously worked on both Buffy the Vampire Slayer and its spin-off show Angel, described Whedon as "casually cruel".
"He has created hostile and toxic work environments since his early career. I know because I experienced it first-hand. Repeatedly," the now 50-year-old wrote.
In her statement, Carpenter claimed Whedon made regular threats to fire her, berated her over her appearance, including calling her "fat" when she was four months pregnant, and pitted cast members against one another.
The actor also claimed that Whedon asked her if she was "going to keep it" when she found out that she was expecting a baby.
"In a closed-door meeting, he asked me if I was 'going to keep it' and manipulatively weaponised my womanhood and faith against me," she claimed.
"He proceeded to attack my character, mock my religious beliefs, accuse me of sabotaging the show, and then unceremoniously fired me the following season once I gave birth."
Since releasing her statement, Carpenter's former co-stars have shared their support on social media.
"While I am proud to have my name associated with Buffy Summers, I don't want to be forever associated with the name Joss Whedon," Sarah Michelle Gellar, who played Buffy, wrote on Instagram.
"I will not be making any further statement at this time. But I stand with all survivors of abuse and am proud of them for speaking out."