The source said the trade will become official Thursday.
According to sources, the trade will involve a 2012 sixth-round draft pick and a conditional sixth-round pick for 2013 going to Washington. The Redskins had wanted fifth- and seventh-round picks from Minnesota.
McNabb had to agree to restructure the five-year, $78 million deal he signed with the Redskins because the Vikings don't have enough cap room for him. The terms of his new deal were not available.
McNabb's arrival in Minnesota has been a rumored scenario for at least five years, ever since Brad Childress left the Philadelphia Eagles to take over the Vikings in 2006.
Childress is long gone now, but another former Eagles assistant -- new Vikings head coach Leslie Frazier -- helped make those rumors a reality on Wednesday, just in time for McNabb to help bridge the gap from Brett Favre to rookie Christian Ponder.
McNabb gives the Vikings a veteran quarterback while they groom Ponder, who was drafted 12th overall out of Florida State in April, to be the team's long-term answer at the position. The original preference for Frazier and the Vikings was to have Ponder start Week 1 in San Diego, with a capable veteran backup there just in case.
That was before the NFL lockout prevented players from working out with coaches all summer and put Ponder behind schedule in his development. That made it more important for the Vikings to add an accomplished, experienced quarterback they think can win games with a roster full of veterans while Ponder gets up to speed.
From the sounds of it, Ponder isn't conceding anything just yet.
"Excited to have McNabb join," Ponder wrote on Twitter on Wednesday morning, perhaps jumping the gun just a little bit. "Will learn a lot from a Pro Bowler. But that doesn't mean I'm not still fighting to start week 1!"
The trade ends a tumultuous one-year run in Washington for McNabb. The 12-year veteran was benched twice last season and threw 14 touchdowns and 15 interceptions in 13 games. He completed 58 percent of his passes for 3,377 yards, and his agent Fletcher Smith sparred publicly with head coach Mike Shanahan and offensive coordinator Kyle Shanahan.
Even before the deal was completed, some Redskins players were already treating McNabb's departure as a foregone conclusion. The writing has been on the wall for some time in Washington, and news of the discussions with the Vikings only served to validate that feeling.
"You see a guy that's been a Pro Bowler six times," linebacker Lorenzo Alexander said. "He was going to come in and really help us win more games, but it didn't work out. Relationships broke down, and now he's not here, but you can't really focus on that. You've got to continue to move forward."
Childress coached McNabb in Philadelphia from 1999 to 2005 before leaving to become the head coach in Minnesota. From the minute he arrived in the Twin Cities, it was assumed that McNabb would one day join him in purple.
But McNabb stayed put in Philly and the Vikings coaxed Favre out of retirement, twice. Favre led the Vikings to the NFC title game after the 2009 season, but had a disastrous, injury and scandal-plagued year in 2010 as Minnesota sunk to the bottom of the division.
When Frazier took over as the full-time head coach in January, he said it was time for the team to end its penchant for bringing in retreads and past-their-prime veterans and develop a young quarterback from the start.
They surprised many when they drafted Ponder so early in the first round and immediately said he would compete for the starting job right away.
Ponder was billed as the most NFL-ready quarterback in the 2011 class, but not being able to work with new offensive coordinator Bill Musgrave and most of his teammates at various minicamps and organized team activities throughout the summer makes it a bigger challenge for him to be ready to start in San Diego on Sept. 11.
Ponder said last week during a workout at the University of Minnesota that he still was aiming to be the starter from Day 1.
"That's what I'm pushing for," he said. "That's what I'm hoping for. So we'll see what happens."
Bringing in McNabb does show veterans such as Adrian Peterson, Antoine Winfield, Jared Allen and Kevin Williams that the team is still in a "win now" mode, and it was unclear on Wednesday night just how the pecking order at quarterback will play out.
"I would have loved to have him back here," Alexander said. "But things just didn't turn out the right way, and he'll go on and eventually be a Hall of Famer."
Information from The Associated Press contributed to this report.