Steven M. Casey, the powerful City Hall figure who has served as Mayor Byron W. Brown’s political alter ego for more than a decade, has resigned as deputy mayor to head the multi-million dollar development proposed for the old Seneca Mall in West Seneca.

Casey said he finalized his long-rumored departure this morning in a meeting with the mayor, informing Brown that he will leave the administration to become chief executive officer of S&R Co. of West Seneca LLC. The firm is a new subsidiary of developer Scott R. Congel’s mall and property development company, which has proposed a major development dubbed “Seneca Place” on the site of the long abandoned Seneca Mall.

Congel’s plan is seen as one of the most extensive private projects in Erie County history, though major questions still surround the idea and West Seneca officials wonder if they can afford to support such a massive undertaking. It features shops, hotels, housing, sports and community facilities and parking, and could ultimately include a new stadium for the Buffalo Bills – though those plans still remain hazy.

Congel’s firm continues to assign a $700 million price tag to the project, though West Seneca officials say its vast scope could still be trimmed.

Nevertheless, Casey said he views Congel’s plan as a “game changer” for the area, and wants to be a part of it.

“I believe in Scott and I believe in this project,” he said. “It’s time to think big. It’s a perfect opportunity and a perfect parcel.”

Congel said this morning he is “thrilled to have a talent like Steve Casey join us.”

“It is unusual to find someone of his range of talents, as he has exhibited in his role as deputy mayor of the City of Buffalo,” he said. “His management experience, understanding of governmental process, integrity and fiscal discipline will be important in our development project. In addition, Steve’s commitment to the growth of the area fits perfectly with our economic growth and jobs creation goals for West Seneca.”

Brown said this morning he is sorry to see his most trusted aide leave.

“I’m very happy for Steve; it’s an incredible opportunity for him,” he said. “I think it’s a recognition of the tremendous talent he has and the effect of what he has done over the course of my three terms in office.”

The mayor said Casey has directed much of city operations in a manner that has allowed “significant” tax reductions that have spurred economic development.

City Hall discussion has long predicted the departure of Casey, who has served as political point man and administration “enforcer” ever since Brown became mayor in 2006. Before that Casey served as a key campaign operative and in Brown’s State Senate office.

The two share a long history that dates to their days in the administration of former County Executive Dennis T. Gorski in the 1990s.

“Obviously, Byron and I are very close friends – personally, professionally and politically,” Casey said. “We’re still very close.”

He called his decision to leave “very difficult,” but also noted that after 23 years in government and at age 47, “it’s time for a change.” He said he looks to bring his administrative experience to the Congel project as its overall manager after helping to direct billions of dollars in new development in Buffalo.

Casey said he will remain a close political adviser to the mayor and will continue his political relationship with lobbyist G. Steven Pigeon, the former Erie County Democratic chairman. Pigeon is also a major figure in the Congel project, serving as its lobbyist and liaison to the town and county governments. He is also working closely with Rochester billionaire B. Thomas Golisano, who has expressed interest in buying the Buffalo Bills.

The Buffalo News reported in May that Golisano and Congel had explored a purchasing partnership for the team, and that Congel has proposed a new Bills stadium in conjunction with his Seneca Place development. But The News last week reported the two are now pursuing their individual interests alone, though some sources say they could still cooperate down the road.

Casey said he believes the site – with access to several major highways and rail – would serve the Bills well.

“I would love to see it happen,” he said. “It’s a perfect site.”

Pigeon – a long time Brown and Casey ally – also said he is looking forward to continuing working with Casey.

“People are moving around, but it’s still the same team,” he said.

Brown also emphasized today that he will continue to work closely with his long time aide.

“One thing I have learned in this business is that you keep your friends and partners close,” he said. “We will continue to have a close relationship.”