McKay was born in Ballymena in 1982, and resides in Rasharkin, County Antrim. He attended St Patrick's Primary School in Rasharkin and St. Louis' Grammar School in Ballymena. Before becoming a Full-time worker for Sinn Féin he lived in Belfast where he worked for the Ulster Bank. Prior to being elected he was involved in aid work in the Palestinian West Bank, and has done work in Cuba, the Basque Country and Kurdistan.
In 2007, it was announced that McKay would contest McGuigan's North Antrim seat in the Northern Ireland Legislature on 7 March 2007 (). He went on to claim his seat after the first count, coming second in the poll behind Ian Paisley of the Democratic Unionist Party (DUP). It was the closest anyone has ever come to beating Ian Paisley in an election in over 35 years. McKay received 7065 votes, compared with Paisley's 7716, and knocked Ian Paisley Junior into third place. He is the youngest MLA in the Assembly.
Since then he has become one of the first Sinn Féin members to sit on the Policing Board along with Martina Anderson and Alex Maskey. He previously sat as Sinn Féin's representative on the Fire and Rescue Services Board.
Mr McKay resides in the village of Rasharkin and has been involved in protests against the Ballymaconnolly loyalist parade through the mainly nationalist village every August for a number of years.
In September 2007 McKay sparked off a political scandal in the Assembly when he named private developer Seymour Sweeney as a member of the DUP in the Assembly Chamber. The DUP's Environment Minister, Arlene Foster, had said that she was 'minded' to approve an application from Mr Sweeney for a private Visitors Centre at the Giant's Causeway even though her Planning Service officials had recommended that it be refused. Mrs Foster threatened to take Mr McKay to court over the matter but in January 2008 she decided to refuse the private application, a move that was welcomed by Mr McKay.
In October 2007 a motion put forward by Mr McKay in the Assembly supporting the closure of the British Nuclear power plant Sellafield was carried, even though both the Ulster Unionist Party and Democratic Unionist Party voted against it.
In 2008 he set up an All-party working group on Climate Change which he currently chairs.
In June 2008 he was one of 4 Sinn Féin Councillors in North Antrim who received death threats.
In July 2008 McKay and fellow Sinn Féin member Cllr Padraig McShane sustained minor injuries in a dispute with local republican youths in Ballymena. The dispute arose because of the youths opposition to the removal of a bonfire to commemorate internment (Operation Demetrius). Mr. McKay said they were in the area ""in opposition to criminal and antisocial elements who are using this bonfire as a cover for other activities". The incident was captured by a BBC camera crew.
The first ever political awards ceremony in the north were organised by Slugger O'Toole in October 2008 and Mr McKay won the Up And Coming MLA of the Year award, beating off opposition from Simon Hamilton (DUP) and Basil McCrea (UUP).