DiNapoli, who lost to Suozzi in a heavily contested 2001 Democratic primary for county executive, said Suozzi will restore fiscal responsibility to Nassau.
Suozzi said DiNapoli is "committed to making government more efficient and ethical. He shares my vision for the future of the county."
Suozzi, who lost to Republican Edward Mangano in 2009, is facing a Democratic primary against East Hills businessman Adam Haber, who said he sought DiNapoli's endorsement. County Democratic leaders back Suozzi.
"After years of scandal-plagued administrations, Nassau families remain frustrated by a government run by political insiders that leave the rest of us overtaxed and underserved," said Haber campaign manager Justin Myers. "Adam Haber has a plan to clean up government and give Nassau the fresh start it needs."
In 2001, DiNapoli, then a Great Neck assemblyman, was the party establishment's candidate while Suozzi, a four-term Glen Cove mayor, was considered an underdog. Suozzi spent nearly $2 million in a campaign that largely avoided personal attacks and defeated DiNapoli by eight percentage points.
DiNapoli, who was elected comptroller in 2007, said he has since become close friends with Suozzi and that "2001 happens to be a long time ago."
DiNapoli is bucking the county Democratic Party in supporting Howard Weitzman as Weitzman seeks to regain his old seat as county comptroller. Party leaders are supporting Legis. Wayne Wink (D-Roslyn).