It lies at the foot of the Apennine Mountains, near the sources of the Sarno River, called Sarnus in ancient times, a stream connected by canal with Pompei and the sea.

Before its incorporation into the domains of the crown of Naples, Sarno gave its name to a county held in succession by the Orsini, Coppola, Tuttavilla, and Colonna families.

[citation needed] The landslides had been caused by several days of torrential rainfalls but were also blamed on agricultural, residential, industrial overexploitation, and the lack of any substantial environmental programs.

The neighbouring bishopric of Cava de' Tirreni was held in personal union with Sarno (united aeque principaliter) from 27 June 1818 till 25 September 1972.

Sarno has the ruins of a medieval castle, which belonged to Count Francesco Coppola, who took an important part in the conspiracy of the barons against Ferdinand II of Aragon in 1485.