He completed his studies in the Colegio San Ignacio and at the Instituto Nacional. Trucco then attended the Universidad de Chile, where he graduated as a lawyer on June 20, 1873.
Early in his life he dedicated most of his time to his Hacienda San José in Puente Alto. Ismael Tocornal married Leonor Cazotte y Alcalde, and together they had one son. Tocornal joined the Radical Party and he was elected supplementary deputy for "Louisiana Victoria" (1879-1882), and reelected, this time as proprietary deputy for the same area (1882-1885).
In 1891 was elected deputy for "Melipilla and Louisiana Victoria" (1891-1894) and in 1897 deputy for "Cauquenes and Constitución" (1897-1900).
He was elected President of the Chamber of deputies on June 9, 1897, position he held until July 2, 1898. President Germán Riesco appointed him Minister of Industry and Public Works, position he held from September 18, 1901 until November 18 of the same year, when he was reappointed as Minister of the Interior, until May 6, 1902.
President Pedro Montt appointed him again as Minister of the Interior between September 15, 1909 and June 17, 1910, same as Presidents Ramón Barros Luco between January 23 and May 20, 1912. Juan Luis Sanfuentes between July 14 and September 11, 1917 and Arturo Alessandri between November 3, 1921 and March 22, 1922.
In 1915, Tocornal was elected a Senator for "Ñuble" (1915-1921).
On June 3, 1918 was elected President of the Senate, position he held until September 8, 1919. In 1919 he was designated as Especial Ambassador to the United Kingdom and headed the delegation that returned the courtesy visit of Especial Ambassador Maurice de Bunsen. In 1920, he was part of the tribunal of honor that decided the result of the presidential election between Arturo Alessandri and Luis Barros Borgoño.
On June 26, 1922 he was once again elected as a Senator, this time for "Santiago", where he remained until 1924.
At the creation of the Central Bank of Chile in 1925, Tocornal was appointed its first Governor, a position he retained until his death in Santiago in 1929, at the age of 79.