The pair faced each other in the Irn-Bru First Division last season when McNamara was at Partick Thistle.
McNamara was all set to emulate Adams in clinching the title this season but left Firhill to take the opportunity at Tannadice.
The 39-year-old former Celtic captain has only lost to his old club and Motherwell since taking charge in January and helped United clinch a top-six place in the Clydesdale Bank Premier League.
Both McNamara and 37-year-old Adams have been tipped as contenders for the manager of the year awards but victory is a must for both men tonight if their teams are to challenge Inverness for a Europa League spot.
Adams, who won the PFA Scotland manager of the year prize last term, said: "It's a good opportunity for him. He had been in the job at Partick Thistle for just over a year, and he got the opportunity to go to Dundee United.
"It was a good opportunity for him, and a very good football club, and I think that he will do well there.
"They have a very good squad, and there is a number of players who have played for Ross County in their time, they have Michael Gardyne, Gary Mackay-Steven and John Rankin that well all welcome back to the football club.
"We enjoy them coming back, but hopefully we can take on our good performance against St Johnstone and possibly win this game."