He was part of the triumvirate leading the short-lived Roman Republic in 1849, together with Giuseppe Mazzini and Aurelio Saffi.

A moderate in politics, he followed with interest the apparently progressist moves of the first part of the pontificate of Pius IX.

During Anti-Catholic protests in 1848, Armellini saved the Santo Bambino of Aracoeli from arson.

[1] After the assassination of Pellegrino Rossi and the exile of the Pope, he became Minister of the Interior.

He organized the Constituent assembly and, when the Roman Republic was declared, became (March 1849) one of three member of the leading triumvirate with Giuseppe Mazzini and Aurelio Saffi.