The DUP peer, also known as Lord Hay of Ballyore, was born near Milford in Donegal.
Lord Hay, who was a former Mayor of Derry, made headlines earlier this year when he won his 20 year battle to be granted a British passport.
He also is a prominent member of the Orange Order and Apprentice Boys of Derry.
Painted in oil on canvas by local artist, Stephen Johnston, the portrait of Lord Hay will join those of his predecessors, Eileen Bell and Lord Alderdice on public display in the Great Hall in Parliament Buildings.
Today’s unveiling ceremony was performed by the Speaker of the Assembly, Edwin Poots MLA.
Mr Poots said he was delighted to unveil the official portrait of Lord Hay in Parliament Buildings.
“Lord Hay was the first Speaker to be elected by the Assembly itself and is the Assembly’s longest serving Speaker, in post from 2007 – 2014,” he said.
“During his tenure, he was passionate about opening up Parliament Buildings to the public and spearheaded a number of important initiatives to encourage local people to actively engage with their Assembly.
“It is therefore very fitting that his portrait will now go on public display in the Great Hall.”