The creation of the Tesorería Mayor de Guerra in October 1703 was one of the most fundamental financial and administrative reforms implemented in Spain during the reign of the first Bourbon king, Philip V. The new department, whose head was Juan de Orcasitas y Avellaneda, second Count of Moriana, was charged with the task of monitoring the military spending of the monarchy, and it was directly subordinated to another new department created in September 1703, the Secretaría del Despacho de Guerra, under the control of Manuel de Coloma Escolano, second Marquis of Canales.

The convenience of creating a central treasury had often been debated in Castile in previous centuries and since the sixteenth century there had existed a Tesorería General in Madrid. Yet, as its ambitious title suggests, the new Tesorería was far from being a truly single treasury. The fiscal incomes collected in the Castilian provinces were gathered in a net of provincial treasuries dispersed over the kingdom. Once there, the provincial treasurers paid the current expenses allocated to them, mainly public debt titles (juros) and payment orders (libranzas). Thus the funds which finally arrived to the Tesorería in Madrid were little more than the remnants of the provincial treasuries. Unlike the Tesorería General, the new Tesorería Mayor de Guerra was devised to fund all war-related expenses of the monarchy, both in the geographical and administrative sense. The military paymasters were transformed into mere subordinates of the Tesorería Mayor de Guerra, so from October 1703 a single royal minister concentrated both funds to pay the military activities of the Crown and its allocation.

Although the War of the Spanish Succession prompted the creation of the Tesorería Mayor de Guerra, it would be wrong to see the new department as a transitional expedient. In reality, it left an unforgettable mark on the later development of Spanish politics and administration. The new institution had to face many problems, proof of which can be found in the fact that the new Tesorería was temporarily suspended twice: between October 1704 and May 1705, and between July 1709 and June 1713.