He graduated as a lawyer from the University of Deusto, specialising in maritime affairs. He started working for one of the major British companies of the time, Harris & Dixon, whose office was located at 21 Grace Church Street in the City of London. As a result of his training and experience, he became a ship owner prior to the First World War. He owned a cargo motor-sailing ship and a cargo steamer called “Anita”.

In 1918, he married Concepción Careaga y Basabe, the daughter of Ambassador Pedro de Careaga y la Quintana, who was later granted the title of Count of Cadagua. A son, Antonio Menchaca Careaga, born in Las Arenas in 1921, was the fruit of this marriage.

In 1921 he purchased two more steamers, the “Nervión” and the “Onsala”, which were used to bring coal for the Briquette Factory in Zorroza, which he established in partnership with the Aznars, next to the wharf on Bilbao’s river. For this same purpose, he purchased a new steamer, which he named “Briquetas de Zorroza”.

Later, he was the owner of the “Banana” and the “Cresalubi”, both ships devoted to Spanish coastal shipping, and also the steamer “Cristina Rueda”.

He was also the owner of the “Antón”, the “Cilurnum” and the “Uribitarte”, the latter two boats were his last ones.

During the Spanish Second Republic, he was appointed Honorary Consul of Peru for Bilbao, the foremost location in the North of Spain, a position for which he earned several medals, and which he held until the time of his death, on 11 June 1968.

He transferred the ownership title of the house of his property, in the municipality of Leioa and named “La Boronita”, to the Religious Institution “Opus Dei”, for use as a school (today´s Gaztelueta School). He also donated one of the family’s properties in Plencia for social activities of the parish church.

He retained this social and philanthropic dimension throughout his life, proof of it was the construction in 1955 of a group of social housing buildings in Leioa, whose project was placed in the hands of Eugenio Mª de Aguinaga.

He also donated to Auxilio Social -an official Charitable-Social institution of the time-, the property called Balneario de la Muera de Arbieto, in a public deed before Public Notary José Pablo de la Herrán y de las Pozas, on 2 August 1943, to be used for the care of children suffering from early tuberculosis, which in those days was a social problem in the province. The donation comprised the La Muera Spa, including the buildings, land and springs, as well as the Hotel, the chapel and a chalet called “Osasuntzua”, the farmhouse “Cuchuelo” and the farmhouse “Arbieto-Goikoa.

In January 1944, he purchased X-ray equipment, which he donated to the Gorliz Sanatorium.

In July 1945, the Council of Getxo, in acknowledgement of Antonio´s ongoing generosity to the Saint Hospital-Municipal Poorhouse, decided at its plenary meeting, and effective from that date, to rename the Avenida de Lejona, to Calle Antonio Menchaca.

In the year 1950, he contributed to the construction of housing for the Centro Obrero de San José de Uretamendi, where workers lived in shacks, by making a cash donation close to 500,000 pesetas.

Among other distinctions, on 4 February 1943, the US Consulate in Bilbao paid tribute to him for the rescue of fourteen shipwrecked sailors of the ship “Alaskan” by the crew of the “Cilurnum”.