At age 25, Vallejo enlisted in the Spanish Colonial army. In 1774, he volunteered for service in Alta California. Vallejo served as a soldier, then an officer, at Mission San Diego de Alcala, and was cited for bravery during the November 1775 indigenous uprising.  

During his 50 years of military service, Vallejo served in San Diego, Monterey, Soledad, San Jose, Santa Cruz and San Luis Obispo. He also served as military escort for Father Junipero Serra, and was with Serra when he came to California to do mission work with the California Indians. Vallejo became a civil engineer in the San Diego area, designing many of the irrigation works of the missions and towns, while becoming an owner of extensive and valuable estates.

Ignacio Vallejo’s qualifying DAR service is for aid rendered in support of the Patriot cause. From 1780-1784, Spanish King Carlos III asked his subjects to donate prayers and money in support of American independence (which limited British encroachment into Spanish California). The Royal Order of August 17, 1780, fixed contributions at two dollars per Spaniard, with each Indian vassal to donate one dollar. Ignacio Vallejo, then a majordomo at San Carlos, donated $10. After a long, active lifetime, he was buried at Mission San Carlos Cemetery in Monterey, California.