The 33-year-old Green was batting .283 with 11 home runs and 51 RBI in 115 games for Arizona this season.
"We're very happy to add Shawn Green to our club and our lineup," Mets GM Omar Minaya said. "He's been a guy I've watched and liked over the years back when he was with Toronto and wirth the Dodgers.
"I think adding him to our club from an offensive standpoint improves our club. He's the kind of player that I think will fit into what we're trying to accomplish here."
A two-time All-Star, Green was informed of the trade before Arizona's game at San Francisco and is expected to join the Mets in time for Thursday's game against the St. Louis Cardinals.
"He's a Met," Minaya said. "I spoke with him and he's very happy to be here," "He had to waive his no-trade clause to be here. ... He's really interested in coming to New York."
Green can look forward to re-acquainting with first baseman Carlos Delgado and catcher Paul Lo Duca, both former teammates.
"He's a great guy, a great player," Delgado said. "I think he can really help us. I hope he helps us down the stretch."
Mets manager Willie Randolph said he believed Green would have little trouble fitting into the Shea Stadium lifestyle.
"He's a solid player," Randolph said. "[For Green] to get around Lo Duca and Delgado, that can sometimes get you kick started."
The Mets could use help from Green, having watched their outfield offensive production dwindle in the weeks following the July 31 non-waivers trade deadline, when right fielder Xavier Nady was sent to the Pittsburgh Pirates in exchange for pitchers Roberto Hernandez and Oliver Perez.
That deal was made necessary when setup reliever Duaner Sanchez separated his right shoulder in a car accident near Miami, ending his season.
Left fielder Cliff Floyd has been on the disabled list since Aug. 9 with tendinitis of the left Achilles, and only reported to the Mets' training facilities in Port St. Lucie, Fla. on Monday following a death in the family.
In the absence of Floyd and Nady, who opened the season as New York's corner outfielders, manager Willie Randolph has mixed and matched with an assortment that has included reserve outfielder Endy Chavez, rookie Lastings Milledge and veteran call-up Michael Tucker.
Green will now be able to take a majority of those at-bats.
"We're talking about a proven veteran hitter here," Minaya said. "The truth is, he doesn't have the power he's had in the past. But the fact is he's a professional hitter."
MacLane, 23, was in the midst of a successful season at Triple-A Norfolk of the International League. Regarded as a control pitcher with the best change-up in the organization, MacLane was 9-8 with a 3.86 ERA in 20 starts at Norfolk.
He started the year at Double-A Binghamton of the Eastern League, going 3-1 with a 4.64 ERA in six starts before being promoted.
According to the Associated Press, Arizona compensated the Mets with more than $6.3 million to offset Green's contract. Green is owed about $12.75 million through 2007.
To make room for Green on New York's 40-man roster, the Mets designated outfielder Victor Diaz for assignment.