In the second and third century, the Roman camp of Lauriacum, in which up to 6,000 soldiers were stationed, was located on the site of modern Enns.

During the Diocletian Persecution of Christians, a commander of the Roman army, Saint Florian, died as a martyr at Lauriacum on 4 May 304, when he was drowned in the Enns river.

The surrounding settlement prospered from the 12th century onwards, when Ottokar II, Margrave of Styria established a market here.

In 1186 the Georgenberg Pact was signed, an inheritance contract between Ottokar IV, Duke of Styria, who lacked a male heir, and the Babenberg duke of Austria, Leopold V. Following the death of Ottokar IV in 1192, his Duchy of Styria — then significantly bigger than the contemporary state, reaching from present day Slovenia to Upper Austria — fell to the House of Babenberg.

[4] Leopold Löw theorized that the Yiddish term "schlemiel" (fool) originated in mockery of Shlomiel of Enns, a 14th-century rabbi.

[5] According to Yaakov Levi Moelin (c. 1365–1427), "Shlomiel of Enns once went to study abroad, and his wife gave birth eleven months later.

After obtaining confessions through torture, Albert II seized all Jewish property in Austria and redistributed it to Christians.