The MLA for East Antrim was speaking as Queen Elizabeth II surpassed the record set by Queen Victoria, her great-great-grandmother.
He said: “This is an incredible milestone in the reign of Queen Elizabeth and on behalf of the Alliance Party I want to offer my congratulations.
“Since 1952 she has been a constant factor in the lives of everyone across the UK, giving hope to many during some of the darkest hours of history.
“Perhaps we should also be celebrating the achievements of a remarkable woman. At 89 years of age she is indomitable in her spirit and has time for everyone whom she speaks to.
“I had the great privilege of meeting Her Majesty at a reception in Buckingham Palace earlier this year, and it was incredible to watch how she met people and the amount of time that she gave individuals to listen to what they were saying to her, and to note that, at a very late hour, she was still ably engaged in conversation.
“Not only do I congratulate her today, but wish her good health and happiness for the future.”