"Over the past weekend, I notified you of recent events that will require my immediate and personal attention," Castro-Wright said in a letter to MetLife Chief Executive Officer Steve Kandarian, a copy of which was filed with securities regulators.
"Accordingly, I now must focus my energy in spending personal time with my family and in protecting my good name and business reputation," he added at the end of the letter.
Castro-Wright was named in a weekend New York Times report as a key figure in an alleged foreign bribery scandal at Wal-Mart.
Castro-Wright, who joined the board in March 2008 and whose term was not due to expire until 2014, sat on three committees: investment, compensation and governance and corporate responsibility.
His 2011 compensation was $259,124 (160,537 pounds) in cash and stock awards, according to MetLife's proxy. The company also paid premiums on a $200,000 life insurance policy for him. He held nearly 17,000 MetLife shares as of March 1, the proxy noted.
MetLife shares rose nearly 0.5 percent to $35.55 in late trading. The shares rallied in the minutes after the company announced Castro-Wright's departure.
Reporting By Ben Berkowitz; Editing by Gary Hill; editing by Carol Bishopric