Historians have found references to ninja activity as early as the 12th century and in the Nanbukucho (1336-1392) and Onin Wars (1467-1477), according to google
Ninjas, or shinobi, emerged in Japan during the 14th century, primarily as covert operatives for feudal lords (daimyō). Their roles included espionage, sabotage, and assassination, particularly during the tumultuous Sengoku period (15th to early 17th centuries) when warlords vied for power. Ninjas were often recruited from lower social classes and were trained in various martial arts, survival skills, and deception techniques, which included disguising themselves as commoners to gather intelligence.
The Iga and Koga regions became renowned for producing skilled ninjas, with specialized training camps established to hone their abilities in stealth and combat. Notable historical accounts document their involvement in significant military operations, such as the burning of Sawayama Castle in 1558.
As Japan transitioned into the Edo period (1603-1868), the role of ninjas diminished due to relative peace, leading many to adopt other professions. However, the romanticized image of ninjas—clad in black and performing extraordinary feats—was popularized through literature and theater during this time, shaping their legacy in modern culture.