Guy V. Molinari, the longtime Republican kingpin of Staten Island who represented New York City’s least populous, most conservative county as an assemblyman, congressman, borough president and political paterfamilias, died on Wednesday in a Manhattan hospital. He was 89 and had been a lifelong resident of the island. His son-in-law, Bill Paxon, said the cause was pneumonia.

Mr. Molinari was a dominant force on Staten Island for two decades, but his influence also reached City Hall, Albany and Washington in Republican alliances with Mayors Michael R. Bloomberg and Rudolph W. Giuliani, Gov. George E. Pataki, and Presidents Ronald Reagan, George Bush and George W. Bush.