The 39-year-old former Celtic, Wolves, Aberdeen and Falkirk defender is set to be unveiled as the manager of SPL side Dundee United today after the Tannadice club met the compensation release clause in his contract.
McNamara has beaten Steven Pressley and Derek McInnes to the United top job after succeeding Ian McCall at Firhill for an 18-month spell in charge.
He leaves the club in second place in the Irn-Bru First Division and having guided them to their first cup final for 41 years as they prepare to face Queen of the South in the Ramsdens Cup Final in April.
McNamara's assistant, his former Celtic team-mate Simon Donnelly, is set to follow him to Tayside.
It is just six weeks since McNamara agreed a new one-year rolling contract with Thistle, a move that saw the former Scotland international rewarded for his fine work in the dugout and the club enhance the release clause in his deal to warn off any potential suitors.
United chairman Stephen Thompson was the first to test Thistle's resolve and has been successful in his pursuit of McNamara, who will replace Peter Houston in the Tannadice hotseat after he parted ways with United this week rather than in the summer as initially planned.
A statement from Thistle read: "Following an approach from Dundee United, the Board of Directors of Partick Thistle Football Club have reluctantly granted permission for Jackie McNamara and Simon Donnelly to speak to them about their vacant managerial position."
The loss of their hugely influential management duo is a significant blow for Thistle, who have been one of Scotland's standout teams this season as they have manoeuvred themselves firmly into title contention aiming to end their exile from the SPL.
McNamara was credited with building one of the most exciting Jags squads the Firhill crowd have seen in many years, with a host of Thistle stars taking to Twitter last night to pay tribute to their former boss.
Defender Conrad Balatoni tweeted: "As boys have said gonna be a big loss is the gaffer.. Helped me alot! But we will continue and do all we can to win this league! " while forward Steven Lawless said "Gutted to see the gaffer go!!! Great to play for. Wish him all the best at united!"
Sean Welsh, who McNamara signed from Hibernian on loan last season and then permanently in the summer tweeted: "Gutted to see the gaffer go!! Wish him all the best with Dundee utd am sure he will do a great job! Great guy and gaffer!! It's time for all the boys to stick together. And win this league & cup for ourselves and the fans!"
Jags stalwart Paul Paton tweeted: "Absolutely gutted the gaffers left. Been great for me and for the club, unbelievable working under him but it's gone now and we move on. Would love Archie to get the job, if he wants it....."
With McNamara set to assume the United reins today and start work preparing for a baptism of fire against Rangers in the Scottish Cup this weekend, the Jags board will soon turn their attentions to appointing his successor.
Archibald is expected to take the side for their crucial Cowdenbeath clash this weekend but the Firhill hierarchy will then have to decide whether to appoint the 35-year-old until the end of the season or invite applications to fill the position and find a man capable of delivering a league and cup double this term.
The club will also want to tie down a number of their most important stars on new long-term deals with key men such as Paton, keeper Scott Fox, defender Stephen O'Donnell and forwards Chris Erskine and Kris Doolan all out of contract come the end of the season and able to speak to other clubs.