Later, he joined the Coquimbo Trade Institute, where he graduated as a General Accountant in 1939, being awarded the best student by the Chamber of Commerce.

Mercado joined the Foreign Relations, Mining and Industry, Interior Police and Regulations, Agriculture and Colonization and Special Housing Committees.

In 1961 he was elected Member of Parliament by the departmental grouping of La Serena, Coquimbo, Elqui, Ovalle, Combarbalá and Illapel, a position which he was occupying since 1958.

During this period he was a member of the Radical Parliamentary Committee and traveled to Tacna and Arica to the Chile-Peru Interparliamentary meeting held in 1965.

In 1969 he founded the Radical Democratic Party along with former senators Julio Duran Neumann and Angel Faivovich Hitzcovich and former Parliamentary representatives Raul Morales Adriasola, Hector Campos Perez and Rafael Señoret Lapsley, among others.

He was elected Parliamentary representative for the departmental group of La Serena, Coquimbo, Elqui, Ovalle, Combarbalá and Illapel, for the period 1969–1973.

Mercado belonged to and directed several trade union, social, cultural and sporting institutions especially the Rotary Club of Coquimbo, serving for years as its president.