The 44-year-old founded Fripps Farm in Great Dunmow, Essex, in 2020, which is home to more than 400 animals.
She said she was "living in my dream" running the business and was the "happiest" she had ever been.
Ms Marsh said OnlyFans was "amazing" and "paid my staff wages for the first year".
"They all joke and say 'your boobs pay our wages'," she told BBC Essex.
"It costs us about £16,000 a month to run this place and that's with everything, with the vet bills, paying staff to be here full time, all the animal feed, bedding and all the bits they need, from medication to the farrier cutting their feet.
"We basically have to find that money every month."
OnlyFans is a social media platform for over 18s where users - including musicians, actors and other celebrities - charge a subscription fee for their content.
The material often includes nudity or is sexually explicit.
Ms Marsh, who posed topless in her previous career, has since spoken publicly about feminism and her experiences being bullied at school.
Ms Marsh said becoming a model was never on her "road map", but she always knew she would open up a sanctuary.
"I see this as a privilege and an honour to help animals and earn their trust and to make them happy," she said.
She added: "I got bullied really badly for being ugly and that's when my stupid teenage brain went 'well I'm going to be a model then, to prove I am not ugly', which is how I ended up in modelling.
"But I am living in my dream.
"Don't get me wrong, all the showbiz stuff was fun and I had a great time and I've got some wonderful memories and even better stories.
"But actually this is all I've ever wanted to do and this is the happiest I've ever been."
Fripps Farm Animal Rescue - a community interest company - homes animals that include donkeys, alpacas, rabbits and turkeys.
It even has a shonkey - a cross between a Shetland pony and a donkey.
