Lagan Valley MLA Basil McCrea, the leader of the NI21 party, is to leave politics after nine years in the Assembly.
Mr McCrea said he had become disillusioned with politics and said he would not contest the assembly election.
He is seeking a new career in business or public life.
Mr McCrea quit the Ulster Unionist Party in 2013, then set up NI21 with South Down MLA John McCallister.
In May 2014 Mr McCrea was accused of sexual harassment but earlier this month the assembly standards commissioner dismissed all complaints of misconduct brought against Mr McCrea.
However, standards committee MLAs said the manner in which he occasionally treated his staff fell short of the standard they would encourage.
Mr McCrea claimed the allegations were a conspiracy to force him to resign.
Speaking to the Belfast Telegraph, this week Mr McCrea said: “Over the past 10 days or so a number of family members were upset by the reality of politics in Northern Ireland.
“Personally, I can take the rough with the smooth, but no man is an island and you have to take on board what family and friends are feeling.”
He added: “There’s no doubt the pressure over the past two years was constant and at times severe.
“It did have an effect on my health, my relationships and my general wellbeing.”
Mr McCrea unsuccessfully contested the 2005 UK General Election in Lagan Valley for the Ulster Unionist Party (UUP).
He was elected in 2007 to the Northern Ireland Assembly as a UUP member for Lagan Valley.
He was a UUP Councillor on Lisburn City Council and was also a NI21 party officer and spokesman for the Northern Ireland Manufacturers’ Focus Group.
McCrea was born in Ramelton, in Donegal.
He was educated at Belfast Royal Academy before attaining a Chemical Engineering degree from the University of Birmingham and a degree in Advanced Computer Technology from the University of Ulster.