The Northern Ireland Assembly has its first republican speaker, after the DUP backed Sinn Fein's Mitchel McLaughlin for the post.
Peter Robinson said his party would support Mr McLaughlin's election because Sinn Fein had honoured its obligations on welfare reform.
Until now it has always been held by unionists or Alliance Party members.
Mitchel McLaughlin is the MLA for South Antrim and is originally from Londonderry.
He has been a close associate of Martin McGuinness since the late 1970s. He was a major contributor to the Sinn Fein document Towards A Lasting Peace In Ireland, which paved the way to ending violence.
Receiving applause as he took his place, Mr McLaughlin vowed to uphold the impartiality of his position.
He said: "I would say to all members, those who supported me and those who did not, I am conscious that I am here to uphold the impartiality and the independence of the office and the interests of this house on behalf of all of you.
"I know there are times when I will have to make judgments which will not please everyone. But I am also as focused on that as I am on what I might be able to do to help increase understanding and agreement both inside and outside this chamber."
Mr McLaughlin replaces the DUP's Willie Hay. He retired in October after seven years because of heart problems but acceded to the House of Lords and is expected to take his seat later this week.
The Democratic Unionists had been delaying the appointment of a successor until the future of the Assembly was secured through a deal on welfare reform, the budget and reform of the institutions at Stormont.
The DUP did not nominate anyone to oppose Mr McLaughlin, but the Ulster Unionists put forward Roy Beggs and the SDLP nominated John Dallat.
Sinn Fein is now expected to support a DUP deputy Speaker.
Martin McGuinness claimed the election would send a strong message about inclusiveness.
He said: "Mitchell and I have been friends for 40 years. I know his ability, dedication and thoughtfulness of his approach to political life.
"I believe him to be well suited to fulfil the onerous responsibility of being Speaker of this Assembly.
"I believe that Mitchel McLaughlin will, as William did, win the respect and admiration of the whole house."
'No party instructions'
Mr Robinson issued a warning that party political connections should be cut: "There should be no party instructions to a Speaker. The Speaker must act independently in that office and I trust that is what will happen."
When devolution was set up under the St Andrews Agreement in 2007 the DUP and Sinn Fein agreed to rotate the post of chair.
Today's election ends a long-standing impasse between Sinn Fein and the DUP and will be seen by many as a sign that the two parties are learning to work together.
However some of the smaller parties, including the SDLP and Ulster Unionists, hit out at what they described as a power carve-up.
Alasdair McDonnell, who leads the SDLP, accused the two largest parties of "re-concocting secret arrangements".
Dr McDonnell said: "It is time we moved to an open and transparent system of government here without backdoor deals or side deals designed to shut out full inclusion and full democracy."
Ulster Unionist leader Mike Nesbitt, who nominated Roy Beggs as a candidate, claimed election of a Sinn Fein Speaker had been a "side deal" from the Stormont House Agreement which was concluded on December 23 following 11 weeks of negotiations.
But the most scathing criticism came from Traditional Unionist Voice (TUV) leader Jim Allister, who said politicians should "hang their heads in shame".
The North Antrim MLA said: "What we are seeing today is the first downpayment by the DUP to Sinn Fein arising from the Stormont House Agreement - the delivery of this side deal.
"Three months ago Mitchell McLaughlin was unelectable. As far as I am concerned he is still unelectable because he is the same Mitchell McLaughlin who, with great notoriety, told the general listening public that that most cruel of crimes - the kidnapping, the murder and secret burial of Jean McConville - was not a crime.
"Yet there are some in this House think someone of that mentality should be made Speaker of this House."
Additional reporting by PA