In an emergency meeting, the Northern Ireland Assembly first voted to restore devolved government and then appointed Ms O’Neill as First Minister.
The meeting comes after the Democratic Unionist Party (DUP) announced late last month that it supports a deal with the UK government and ends a long-running boycott of a devolved government in Northern Ireland.
In February 2022, the DUP quit the provincial government in protest at post-Brexit trade deals between London and Brussels for the only UK region with a land border with the European Union.
The Assembly also appointed the DUP’s Emma Little-Pengelly as Deputy First Minister.
Ms O’Neill, 47, has been waiting patiently to take up the post after Sinn Fein won enough seats in Northern Ireland’s parliament in May 2022 to appoint a politician as first minister.
“This is a historic day and a new dawn,” Ms. O’Neill told fellow lawmakers shortly after her appointment.
“The past cannot be changed or erased, but what we can do, what we can all do, is to create a better future,” she added.
One of her first tasks is to solve the budget crisis and the problem of collapsing public services.