He was intended for the legal profession, but his passion for architecture was too strong, and after holding some government posts at Milan, he entered as a competitor for the construction of the Porta Orientale.
After the death of his father he spent two years in Verona and Venice, studying the architectural structures of these cities.
In 1806 he was called upon to erect a triumphal arch for the marriage of Eugene Beauharnais with the princess of Bavaria.
The result was the magnificent Arco della Pace ("Arch of Peace") at Porta Sempione in Milan, surpassed in dimensions only by the Arc de Triomphe in Paris.
Among other works executed by Cagnola are the Porta Ticinese and the chapel of Santa Marcellina in Milan, and the church tower in Urgnano.