Hacham Yaakov Shaul Elyashar, son of Rabbi Eliezer Yeruham Elyashar, was born in Safed in 1817. His father was the city's leading shochet and bodeq [inspector] and also served as a dayan and as the cantor for the city's Kushta community.
In 1824, when Hacham Yaakov Shaul Elyashar was seven, his father died. His mother, suffering from poverty, was forced to sell her home and belongings, and supported her only son by working as a seamstress.
In 1828, his mother married Hacham Binyamin Mordecai Navon, head of the Jerusalem chief judges of the rabbinic courts, who adopted Hacham Yaakov Shaul Elyashar and was like a father, teacher and rabbi to him. Hacham Yaakov Shaul Elyashar was already considered a Torah prodigy when he reached Bar Mitzvah age, yet he chose to earn his living from commerce.
In 1832, at the age of fifteen, he married a girl who had been orphaned, as he had. Their eldest daughter and first two sons (a third was subsequently born) were born while the couple was still living in Hacham Binyamin Mordecai Navon's home.
In 1853, he was ordained as a dayan and Halachic adjudicator by the Jerusalem rabbis and sent as the Jerusalem Sephardi community's emissary to Alexandria to annul the Alexandria community's decision to cease receiving rabbinic emissaries from the Land of Israel. Hacham Yaakov Shaul Elyashar succeeded in having the decision annulled, and was asked to remain in Alexandria as the city's rabbi, but refused the proposition.
In 1869 he was appointed President of the Jerusalem Rabbinic Court, replacing Hacham Abraham Ashkenazy who had been accepted to the post of Rishon LeZion. In 1893, following the death of Hacham Raphael Meir Panigel, eldest of the Jerusalem rabbis, Hacham Yaakov Shaul Elyashar was instated as Rishon LeZion. He presided in this role for thirteen years, until his death on 18 Tammuz 5666 (1906). Hacham Yaakov Shaul Elyashar was buried in the Mount of Olives cemetery. Four years after his death, in 1910, his son Hacham Nissim Elyashar founded Jerusalem's Givat Shaul neighborhood in his name.
Hacham Yaakov Shaul Elyashar authored many books, including several books of sermons – ISH Emunim, Derech ISH, Divrei ISH and 'Olat ISH; his books of Responsa include Simcha le'ISH, Ma'aseh ISH, She'al HaIsh and Pnei Hama (which includes innovations by his son); Bnei Binyamin, Krav ISH – contain complementary material and innovations on the book by his adoptive father. He authored several additional books and piyutim.