The 31-year-old Benson spent parts of five seasons with the Pittsburgh Pirates before being traded to New York in July 2004. He missed the entire 2001 season after elbow ligament replacement surgery on his right elbow.
For his career, he is 57-61 with a 4.25 ERA in 165 starts.
"I was shocked when I heard about the trade this morning, but the more I thought about it, the more excited I got," Benson said. "I'm looking forward to helping out the rotation and doing everything I can to make this team a contender."
Mets general manager Omar Minaya said Benson's wife, Anna Benson, was not a factor in the trade. A model and an actress, she has posed topless and publicly discussed intimate details about their sex life.
"We were able to get a power arm on the back end of the bullpen and we were able to get a young player, Maine, who had pretty good numbers in the minor leagues," Minaya said. "It gives us some depth. Now Aaron Heilman can get an opportunity to start."
New York's projected starting rotation is headed by Pedro Martinez, Tom Glavine, Victor Zambrano and Steve Trachsel. Heilman and Maine are candidates for the fifth spot along with rookie Brian Bannister, Cuban defector Alay Soler and Japanese right-hander Yusaku Iriki.
Julio, a hard thrower, had 83 saves from 2002-04 and joins Duaner Sanchez in the set-up spot for new closer Billy Wagner.
The deal also saves New York about $5 million this year. Julio, who is in salary arbitration, will make about $2.55 million. Benson is owed $7.5 million in each of the next two seasons, and Baltimore inherits a $7.5 million option for 2008 with a $500,000 buyout.
Benson expressed excitement about the prospect of working with pitching coach Leo Mazzone, throwing to catcher Ramon Hernandez and working at Camden Yards.
"I've heard good things about their fan base," he said.
The Orioles have been trying for two seasons to add a veteran starting pitcher to the rotation, and vice president Jim Duquette said Baltimore targeted Benson soon after the 2005 season ended.
"Our pursuit of Kris goes back to November," Duquette said. "One of our goals going into the offseason was to acquire another pitcher."
Orioles executive vice president Mike Flanagan said, "Jim and I have been targeting people like Kris for veteran leadership. ... Obviously we're very pleased to add a pitcher of Kris Benson's ilk to our rotation."
Julio was Baltimore's closer from 2002-04. He had a 1.99 ERA in 2002 and a career-high 36 saves in 2003, but slipped the following season and ultimately lost the closer's job to B.J. Ryan. Working primarily as the setup man in 2005, Julio went 3-5 with a 5.90 ERA in 67 appearances last season.
In five major league seasons, he is 11-24 with a 4.20 ERA.
Julio, 26, became expendable after the Orioles acquired LaTroy Hawkins in a trade with the San Francisco Giants.
Maine went 2-3 with a 6.30 ERA in 10 appearances for the Orioles last year after being recalled from Triple-A Ottawa in August. He was the Orioles' sixth-round pick in the 2002 draft.
Notes: Martinez's injured toe is improving. "Pedro's been feeling much better based on what the agent told me yesterday," said Minaya, who spoke Friday with Fernando Cuza.
Associated Press writers Ronald Blum in New York and Ben Nuckols in Baltimore contributed to this report.