According to the New York Daily News' Andy Martino, the extension will be for three years, though some details remain to be decided.
Alderson took the position after the end of the 2010 season, during which the team went 79-83, notching a .488 winning percentage and finishing 4th in the NL East. In the first three seasons under Alderson, the team won 77, 74, and 74 games, never finishing higher than 3rd place.
During Alderson's tenure, the Mets' team salary has shrunk to approximately $83 million this year, ranking in the bottom third of the league.
New York is 76-80 and has been eliminated from playoff contention.
Adam Rubin of ESPN New York reported on Monday that manager Terry Collins would return, as well.