He studied in a Jesuit school and at age 18 he joined the Spanish utility company FENSA which would later become Iberdrola.
Through this position, he defended the social and labor rights of all the Navarrean workers in opposition to the official union that Francoist Spain had enabled after the Spanish Civil War.
In 1976, he was appointed mayor of Pamplona as the transition to democracy had been initiated in Spain and 1977[3] he resigned this position in order to run for congressman in the first Spanish General Election as a candidate for the Frente Navarro Independiente.
In 1995 he was elected to the Pamplona city council under the Unión del Pueblo Navarro (UPN) candidate list although he ran as an independent.
Tomás eventually joined UPN and took over the previous spokesman for the local government, Santiago Cervera.
On 1998, three political parties in the Pamplona city council (PSN, CDN and IU) forced UPN into the opposition benches.
On May 6, 1998, Tomás Caballero was assassinated by ETA members at the doorstep of his house as he was getting ready to leave home for work.
The fact that Tomás had led a life dedicated to the protection of the rights of workers and the promotion of a more plural, diverse, and democratic Navarre and Spain.