Agnello Participazio had taken up the defense of Venice during the subsequent siege of Pepin of Italy in 810 after the city’s reigning doges, Obelerio, Beato, and Valentino, had called in Pepin to support their tottering power.

Following the flight of Pepin and exile of the doges, Agnello is elected to the dogeship in 811.

Pro-Greek reaction to the Veneto-Frankish alliance under the doges of the Parteciaco family leads to the transfer of the seat of government from the island of Malamocco (now Lido) to the Rialto group of islands, by now the center for exiles in the factional fighting, and where where political independence could be more easily maintained.

Agnello Participazio’s Rialtine house on the Campiello della Cason becomes the first doge's palace in Venetian history and he soon begins expanding it in stone.

However, no traces remains of that ninth century building, owing to the partial destruction of the palace by a fire in the tenth century.

His reign begins on a happy note.

By the Pax Nicephori, Venetia is retained by the Constantinople and renounced by Charles I of the Franks, but its practical independence is affirmed.

Though the Franco-Greek treaty of 814 guarantees to Venice political and juridical independence from the rule of the Western Empire, it does not confirm any effective dependence on Constantinople.

Agnello begins the minting of the first Venetian coinage.