Wagenen is the same agent who represents some of Lannan's teammates, including third baseman Ryan Zimmerman, right-hander Drew Storen and outfielder Justin Maxwell.
Lannan made the decision to drop Mongelluzzi in January, but the dismissal was not known until Monday.
Van Wagenen, who was spotted at the Nationals' Spring Training complex on Monday, declined to comment, but Lannan said he needed to be represented the right way, because he will be eligible for arbitration after the 2010 season and become a free agent three years later.
Lannan called his decision to switch agents a tough one.
"I just heard great things [about] Brodie Van Wagenen," Lannan said. "I've seen him around. I met with him and I like the way [CAA] handled their business and represented their clients, and that was what I was looking for.
"It was definitely a business decision and I'm glad I made the move. I'm confident with my representation for the future, arbitration, free agency and all that other kinds of tough [stuff].
Lannan was clearly the Nationals' best pitcher last year, going 9-13 with a 3.88 ERA. There's still room for improvement, however. Lannan indicated that he must get better against left-handed hitters and go deeper into games.
Left-handed hitters were 56-for-193 (.290) with nine home runs and 28 RBIs against Lannan, while he averaged 6.24 innings per start.
In order to get left-handed hitters out, Lannan must throw his four-seam fastball and slider down and away.
"A big part of my game is getting lefties out," Lannan said. "That batting average was too high. Just little things here and there. I'm basically going to stay the same, make my pitches and keep my delivery the same."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.