Lannan last pitched in the majors for the Mets in 2014 and spent last season at Triple-A Omaha, the top Royals affiliate, where he went 7-8 with a 5.24 ERA in 25 games (19 starts).
His deal does not include an invitation to major league camp.
The 32-year-old Lannan was an 11th-round choice of the Nationals in the 2005 First-Year Player Draft out of Siena. He debuted in the majors two seasons later, going 2-2 in six starts.
In six seasons with Washington, Lannan went 42-52 with a 4.01 ERA in 134 games, all starts. Lannan was the Nationals’ opening day starter in 2009 and 2010.
Update: Lannan wants to reinvent himself as a sidewinding left-hander working out of the bullpen, according to a source.