The Union flag will fly at half-mast over Stormont's Parliament Buildings on the day of Margaret Thatcher's funeral.
A statement from the Assembly came after Traditional Unionist Voice leader Jim Allister raised the issue directly with Speaker William Hay.
Mr Hay also chairs the Assembly Commission, which is responsible for the day to day running of Parliament Buildings and will oversee the display.
The statement said the decision adheres to the Flags (Northern Ireland) Order 2000, which was amended by regulations which specify the days on which the Union flag is flown from Government buildings in Northern Ireland – including for the death of a serving or former Prime Minister.
The commission is meeting on Wednesday to finalise arrangements designed to coincide with the other devolved administrations in Edinburgh and Cardiff.
Baroness Thatcher's last act of political choreography will be her funeral, which was agreed with her before her death.
In line with her wishes she will not be given a State funeral, lie in state or receive a fly-past of military aircraft, which she thought would be a "waste of money".
Instead, she will receive a ceremonial funeral with full military honours at St Paul's Cathedral – similar to that afforded to the Queen Mother.
The date of the funeral has yet to be announced. The night before, Lady Thatcher's body will be moved to the Chapel of St Mary Undercroft in the Palace of Westminster, where it will rest following a short ceremony.
The next day she will travel by hearse from Westminster to the Church of St Clement Danes, where her coffin will be transferred to a gun carriage drawn by the King's Troop Royal Artillery. It will then be borne in procession to St Paul's along a route lined by military personnel.
Downing Street said the guest list for the funeral itself would include family and friends of Lady Thatcher's, former colleagues, and representatives of foreign governments.
Her friend and adviser Lord Bell, who announced the news of her death, said: "She specifically did not want a State funeral and nor did her family. She particularly did not wish to lie in state as she thought that was not appropriate."
The costs of the funeral will be borne by the Government and Lady Thatcher's estate, Downing Street said.
Those wanting to commemorate her life are asked to give donations to the Royal Hospital Chelsea.