Mr McNarry was expelled from the UUP earlier this year following a bust-up with then leader Tom Elliott, over comments about Unionist unity.
After revealing his decision to join UKIP as its first Assembly Member, the Strangford MLA sent a stark message to his former party.
"I am a politician, I am a professional politician and as far as I am concerned I am on the way up," said the 64-year-old, who had been in the UUP most of his adult life.
I am not associated with anybody or anything that's in decline, and you best understand that. I'm on the way up.
"I have not defected from anyone, I have willingly joined - the choice was mine."
The UK Independence Party - which advocates withdrawal from the European Union and strict curbs on immigration - is the only national party which contests seats in Northern Ireland, and it put forward eight candidates in last year's Stormont elections.
Among its councillors in NI is former Ulster Unionist Henry Reilly at Newry and Mourne District Council.
Leader Nigel Farage was at Parliament Buildings in Belfast on Friday to welcome Mr McNarry into the party's ranks.
He also confirmed UKIP would stand in the region in the European election in 2014.
Mr Farage said: "I think the fact the Westminster parties have completely given up on Northern Irish politics - completely given up - and the fact that we are now here today, we are sending a very big signal: we are a Unionist party, we are a Unionist party that wants to be involved in all the different corners of the United Kingdom.
"The ambition in 2014 is to come first (in the poll) across England, Scotland, Wales and Northern Ireland."
Mr McNarry's departure from the UUP came after a newspaper interview about "secret talks" on the prospect of his party and the DUP joining forces.
Disciplinary action from Mr Elliott saw him dropped from his position as deputy chair of the Stormont Education Committee, before being subsequently expelled from the party outright.
I have a lot and still have a lot of good friends in the Ulster Unionist Party, the Democratic Unionist Party and in the other parties. But this is politics, this is about channelling your views and getting support from the public for them and that's what I intend to do.
The subject of unionist unity was raised once again this week when current leader Mike Nesbitt sacked John McCallister from his Assembly grouping after interpreting comments he made in a speech as a criticism of his stewardship.
Mr McNarry, who is a proponent of unity, was asked if his decision to bring yet another unionist party - the fourth in addition to the DUP, UUP and TUV - into the Assembly chamber runs contrary to his beliefs.
He responded: "I am a supporter of unionist unity; you will know very well how some people took offence at that.
"Insofar as UKIP's involvement, UKIP are a Unionist party and I have spoken to Nigel and Paul (deputy leader Paul Nuttall) and we will play a full role in that. The key to UKIP's involvement in it is that UKIP are the only Unionist party with a national agenda."
However Ulster Unionist Peer Lord Empey has questioned the move.
He said: "Many people will appreciate the irony of a man who was wedded to the notion of unionist unity and the establishment of a single unionist party between the Ulster Unionists and the DUP, introducing a fourth unionist party into the Assembly in the form of UKIP."
Mr McNarry added that he is confident his constituents will back his decision to switch parties mid-way through the Assembly term.
He said: "I think that my constituents will judge me as they have always judged me - in that, if I'm worthy of a vote, I've got the vote. People accept me as I am."