Guillermo Pereira Íñiguez joined the Conservative Party in 1895 and began a long parliamentary career representing the districts of Ancud, Quinchao and Castro. He was first elected to the Chamber of Deputies of Chile for the 1900–1903 legislative term, during which he served on the Standing Committee on Government and Internal Police. In 1903, he was appointed as a member of the Chilean delegation to the signing of the Treaty of Peace and Friendship between Chile and Bolivia, held in La Paz.[1]
He was re-elected deputy for successive terms covering the periods 1903–1906, 1906–1909 and 1909–1912. During these mandates, he served on several standing committees, including Public Education, Foreign Relations, and Legislation and Justice, and between 1907 and 1909 he held the position of Secretary General of the Conservative Party. In 1910, alongside his parliamentary duties, he founded an agricultural trading company in the Colchagua Valley.[1]
In 1912, Pereira Íñiguez was elected Vice President of the Chamber of Deputies of Chile, holding the position from 7 January to 15 May. That same year, he represented Chile at the centenary celebrations of the Cortes of Cádiz in Spain. He continued to serve as deputy for Ancud and Quinchao during the 1912–1915 and 1915–1918 legislative terms, maintaining his participation in the Standing Committee on Foreign Relations and Colonization, and in 1913 he was appointed delegate to the International Congress of Agriculture held in Ghent, Belgium.[1]
In 1918, he was appointed Minister of Foreign Affairs, while continuing his parliamentary service. He was subsequently re-elected deputy for the periods 1918–1921 and 1921–1924, serving on the Standing Committee on Internal Government and Regulations as well as the Standing Committee on Foreign Relations and Colonization. In 1919, he represented Chile at the embassy in Argentina during the inauguration of the monument to Bernardo O'Higgins in Buenos Aires.[1]
Following the military movement led by General Luis Altamirano, which overthrew President Arturo Alessandri on 11 September 1924, Pereira Íñiguez withdrew from active political life, although he retained an influential leadership role within the Conservative Party. In later years, he served as a director of Banco Popular in 1930, remained active in elite social circles as a member of the Club de la Unión and an honorary member of the Ibero-American Club, and received several honors, including the Cross of St. Gregory the Great awarded by the Holy See, the Gold Medal of the Cortes of Cádiz bestowed by the King of Spain, and the Commander's Plaque of the Order of Christ conferred by the Portuguese government.[1]