Killian, who ran an unsuccessful challenge to Latimer in 2016 will face Democratic Assemblywoman Shelley Mayer of Yonkers during a special election on April 24.
Republican Party members chose Killian, who served six years as a Rye city councilwoman, over former Yonkers inspector general Dan Schorr of White Plains during a small GOP convention at Westchester Manor on Wednesday.
The Senate seat has been vacant since Latimer took over as county executive on Jan. 1 after defeating Rob Astorino in November.
Republicans hold a one-vote majority in the state Senate with two seats open.
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