[en] Mauricio Carlos de Onís began serving as a diplomat during both the Peninsular War and the Liberal Triennium. He was a progressive, liberal politician during the Regencies. His family included both enlightened reformist diplomats, and moderate realists. In 1843, he began to evolve from progressivism to moderate positions. He became a "notable" Castilian and a servant of the Court with aspirations to a nobility title. His evolution was similar to that of other progressives who were part of the Liberal Union at the end of Elizabeth II's reign. Being a personality from the time of the Napoleonic wars and the liberal revolution, it is difficult to classify him as an heir to his relatives' enlightened reformism, thus ascribing to temperate liberalism during the Triennium –and evolving during the Regency of María Cristina towards progressivism, as secretary of the Congress in the constituent legislature. His family ties with the González Salmón and the Heredia –some of whom were presidents of the government– and with the network of political friends with a similar evolution towards doctrinaire liberalism, such as Antonio González and Pedro Pascual Oliver– were crucial in his career. After his promotion to Foreign Minister in 1840, and to the presidency of the Senate in 1843, he favored "legal, moderate progress", distancing himself from both General Espartero's liberal followers and the moderate party. A great landowner in Salamanca, he joined the Court in the entourage of the infant Francisco de Paula Borbón and his son, the king consort. He was also a capitalist investor in mercantile companies and a promoter of public works, presiding over the Sociedad Económica Matritense.

[es] Mauricio Carlos de Onís inició su vida pública como diplomático entre la guerra de la Independencia y el Trienio Liberal, para pasar a ser un político liberal progresista durante las Regencias. Perteneciente a una familia de diplomáticos reformistas ilustrados o realistas moderados, evolucionó desde el progresismo a posiciones más moderadas a partir de 1843, convirtiéndose en un "notable" castellano y servidor de la Corte con aspiraciones a título nobiliario. Su evolución fue similar a la de otros progresistas que formaron parte de la Unión Liberal al final del reinado de Isabel II. Al ser una personalidad iniciada en el tiempo de las guerras napoleónicas y de la revolución liberal, resulta difícil encasillarle como heredero del reformismo ilustrado de sus familiares, adscribiéndose al liberalismo templado durante el Trienio, y evolucionando durante la Regencia de María Cristina hacia el progresismo. Los enlaces familiares con los González Salmón y los Heredia –algunos de los cuales alcanzaron la presidencia del Gobierno– y la red de amistades políticas con progresistas de similar evolución hacia el liberalismo doctrinario, fueron cruciales en su trayectoria. Tras su ascenso a ministro de Exteriores en 1840, y a la presidencia del Senado en 1843, fue partidario de un "progreso legal moderado" distanciándose, tanto de los liberales seguidores del general Espartero, como del partido moderado. Gran propietario en Salamanca, se vinculó a la Corte, formando parte de la clientela del infante Francisco de Paula Borbón y de su hijo, el rey consorte. Al mismo tiempo, fue inversor capitalista en sociedades mercantiles y promotor de Obras Públicas, presidiendo la Sociedad Económica Matritense.