The ban came into force after former leader Ellen Johnson-Sirleaf signed an executive order before stepping down.
The move received little publicity, overshadowed by Monday’s inauguration of Johnson-Sirleaf’s successor, former soccer star George Weah.
The ban makes it an offence to perform FGM on anyone under 18 but it can still be carried out on adults with their consent.
Campaigners said FGM should be banned outright as even women who gave consent often did so under pressure.
“It is too early to celebrate as there is still a long way to go before there is zero tolerance to FGM in Liberia,” said Grace Uwizeye, a consultant with international rights group Equality Now.
Activists have long campaigned for FGM to be outlawed in Liberia, a country of about 4.6 million people, where around half of women have undergone the procedure.
However, FGM has been an awkward issue for Johnson-Sirleaf – Africa’s first female president – because it is overseen by a highly secretive and politically influential women’s society.
Supporters say the ritual, involving the partial or total removal of the external genitalia, is a key rite of passage. But it often causes health problems and can be fatal.
In 2017, parliamentarians removed FGM from Liberia’s domestic violence bill, saying it was a cultural matter.
According to an official statement Johnson-Sirleaf said on Jan. 19 that its omission undermined the law. Executive orders expire after a year.
Campaigners have received death threats for speaking out about the practice which remains shrouded in taboo and is often performed during initiation ceremonies in bush schools with girls sworn to secrecy on their lives.
FGM is practised in nearly 30 African countries, according to UN data, even though almost all have outlawed it.
Mackins Pajibo of Liberian group Women Solidarity Incorporated said the ban was good news, but probably not enforceable.
“We call on President Weah … to keep up the pressure for a permanent law which bans FGM completely for all women and children,” she told the Media.
Lakshmi Moore, acting country director with Action Aid, said there was a lot to do in 2019 to push for a permanent and enforceable ban.
“We know this is still a taboo topic in some communities, but we are seeing signs that attitudes are changing,” Moore said.