As he writes to Mary and Tony, it has been several years since he moved back to NY. Since the movie takes pplace in 1979, that could mean the mid-70s or earlier.

When Michael "retired" from the street action of the Cosa Nostra, he turned the Family over to Zasa. But, he apparently was a member of the Commission, wealthy, respected, and obviously powerful since Altobello was intent on getting rid of him.