The legislation came into force at 00:01.
As of early Tuesday morning, people who seek access to abortion in Northern Ireland will not be prosecuted, investigations of illegal abortions will no longer be pursued, and prosecutions currently underway will be halted.
Under the new legislation, introduced via intervention from Westminster in July, same-sex marriage is also being legalised, and people who were severely physically or psychologically injured in the Troubles are to receive annual pensions of £2,000-£10,000 (€2,326 - €11,630).
The payment scheme covers those injured through no fault of their own. The victims' payment scheme will be introduced by the end of May 2020.
The first same-sex weddings in Northern Ireland are set to take place in February 2020 and the government has until the end of March to come up with regulations for the provision of abortion services.
The law changes were threatened by the presence of the DUP and other unionist parties who triggered a recall of the Northern Ireland Assembly in an attempt to block the lifting of a near-ban on terminations.
DUP leader Arlene Foster said her party would “take every possible legal option open to us”, up to and including repeal, to try and prevent the legalisation of abortion. “These are abnormal days and we are in a very abnormal situation,” she said.
Sinn Féin, Alliance, the Green Party and People Before Profit did not attend the Stormont sitting, which Sinn Féin described as a "cynical political stunt".
Abortion is legal under certain circumstances in England and Wales and Scotland based on the Abortion Act 1967, then one of the most liberal abortion laws in Europe when it was enacted.
However, this act didn't cover Northern Ireland where women were only permitted a termination if their life is at risk or there is a serious risk to their mental health.
This law banned abortion in almost all circumstances including rape, incest and fatal foetal abnormality. It provides for severe penalties including life imprisonment.
Last month, the High Court in Belfast ruled Northern Ireland's abortion law in breach of the UK's human rights commitments.
Northern Ireland Secretary Julian Smith is mandated to put in place regulations by 31 March 2020.
“Free, safe, and legal abortion access across the island of Ireland has been a central goal of our campaign since its foundation in 2012,” said Anna Carnegie, spokesperson for the Abortion Rights Campaign.
“Today, we move one step closer to making that a reality. It’s vital that we now keep the pressure on to ensure accessible services are in place by the March 2020 deadline, otherwise, some people will still be forced to travel, while others may not be able to.
"We must continue to fight for these individuals, and we must keep fighting until free, safe, legal, and local abortion is a reality for all, across all of Ireland.”