Born in the province of Toledo, Contreras joined the Regiment of Alcázar de San Juan as a cadet in 1772, being promoted to lieutenant in 1783.
[2] In August 1800, his division garrisoned Badajoz and Alcántara and saw action in the War of the Oranges, participating at Juromenha and the sieges of Elvas and Campo Maior.
[2] In February 1809, he was given command of the 4th Infantry Division of Cartaojal's Ejército de la Mancha but was transferred to the Army of Extremadura the following April.
[2] In March–April 1810, the Marquis of La Romana sent Contreras's division, together with those of Mendizabal and Ballesteros, to harass the French 2nd Corps (still under the temporary command of General Heudelet) in Andalusia.
[3] Following his defence of Cartagena, he was promoted to field marshal in April 1810 and appointed military governor of La Coruña and commander-in-chief of Galicia.