Born in Minsk in the Russian Empire (today in Belarus), Kaplan attended a heder and high school in Łowicz.
[1] He joined the Socialist Zionist Party in 1905, and was one of the founders of the Youth of Zion – Renewal movement in 1908,[1] later being elected secretary of its Minsk Region branch in 1912.
[1] The following year he immigrated to Mandatory Palestine and was involved in merging Youth of Zion with Hapoel Hatzair to form Hitachdut.
[1] A member of the Assembly of Representatives, on 14 May 1948 Kaplan was one of the people to sign the Israeli declaration of independence, and was immediately appointed Minister of Finance in the provisional government.
He was elected to the first Knesset as a member of Mapai, and retained the Finance Ministry post, also becoming Minister of Trade and Industry in Ben-Gurion's first government.