Born in Minsk in the Russian Empire (today in Belarus), Kaplan attended a Heder and high school in Lubitch. He joined the Socialist Zionist Party in 1905, and was one of the founders of the Youth of Zion – Renewal movement in 1908, elected secretary of its Minsk Region branch in 1912. He also helped found the Youth of Zion movement in Russia in 1912 and was a member of its central committee. In 1917 he graduated from a Moscow polytechnic as a building engineer.
Kaplan returned to Mandate Palestine in 1923 and joined the Histadrut's Office of Public Works. A director of the technical department of the Tel Aviv municipality between 1923 and 1925, he was elected to Tel Aviv city council in 1925, remaining on the council until 1933. That year he joined the board of the Jewish Agency and served as its treasurer until 1948.