The former East Antrim MLA served as Leader of Alliance between 1998 and 2001, continuing as MLA until 2011. He was first elected to the then-Carrickfergus Borough Council in 1977, later serving on it as Mayor.
"My thoughts and prayers today are with Sean's family and friends,” said Mrs Long.
"But today, Alliance has also lost a part of our history. Sean showed leadership at a time when the party needed it, and we will always be grateful to him for that.
"Known for getting things done, Sean was a dedicated public servant who delivered for everyone in Carrickfergus as a Councillor and then the wider East Antrim area when he became an MLA, a role he was delighted to take on.
"In later years, Sean's passion for Alliance and Northern Ireland as a whole continued, and I was delighted he was well enough to join us at Conference just last year."
"My thoughts and prayers today are with Sean's family and friends,” said Mrs Long.
"But today, Alliance has also lost a part of our history. Sean showed leadership at a time when the party needed it, and we will always be grateful to him for that.
"Known for getting things done, Sean was a dedicated public servant who delivered for everyone in Carrickfergus as a Councillor and then the wider East Antrim area when he became an MLA, a role he was delighted to take on.
"In later years, Sean's passion for Alliance and Northern Ireland as a whole continued, and I was delighted he was well enough to join us at Conference just last year."