Lady Hermon was the MP for North Down for 18 years, since 2001.
First elected under the Ulster Unionist banner, she left the party in 2010 following its electoral pact with the Conservative Party.
Lady Hermon said the gulf was too wide between herself and the Ulster Unionist Party (UUP).
But the voters stayed with her and in the 2010 election she retained the seat with 21,181 votes.
The Ulster Conservatives and Unionists - New Force polled just 6,817 votes.
However, in subsequent elections Lady Hermon has faced a bigger challenge from Democratic Unionist Party (DUP) candidates.
In 2015, her vote had dropped to 17,689, with the DUP's Alex Easton coming in second with 8,847 votes.
Two years later, in the general election called by Theresa May, the gap between the two candidates had fallen to just 1,200 votes.
Lady Hermon is a former law lecturer who once worked with David Trimble at Queen's University in Belfast.
A staunch supporter of his pro-Good Friday Agreement stance during his time as UUP leader, Lady Hermon ousted the anti-Agreement unionist Robert McCartney in 2001.
In the party's disastrous 2005 general election Lady Hermon was the only UUP MP to retain her parliamentary seat, taking 50.4% of the vote.
Since the Brexit referendum, she has been the only pro-Remain NI MP in the House of Commons, frequently clashing with DUP MPs and the government during Westminster debates.
In March, Lady Hermon told the House of Commons the DUP "do not speak for the majority of people in Northern Ireland".
Lady Hermon voted against Boris Johnson's Brexit deal in October 2019, asking the prime minister for a "guarantee that there is nothing in this deal, this new Brexit deal, which undermines or weakens the constitutional status of Northern Ireland".
A member of the Northern Ireland Affairs Committee at Westminster, Lady Hermon has in recent years been a vocal critic of Northern Ireland Assembly members retaining their salaries, despite the collapse on Stormont.
In October, she said it was "unsustainable and indefensible" that assembly members were to get their full salaries while "schools and hospitals are under so much pressure".
It was confirmed that £14.9m has been spent on pay since January 2017.
The Secretary of State Julian Smith said the sums of money involved were "unacceptable".