She was born with the name Justina Kristina Wässelius. Her sister, Jeanette Wässelius, also became a singer, but she only worked in Sweden. Justina went to the theatre school Dramatens elevskola in 1805. When she graduated in 1812, she began to work at the theatre Royal Swedish Opera. The same year, she married the Italian dancer Ludovico Casagli, and got the name Justina Casagli.
She left Sweden with her husband in 1818 and went to Italy. In Italy, she played in the play "Cerentola" in Turino in 1818, in the play "Donna del Lago" in Rome in 1823 and in Lucca 1827 in the play "Matrimonio Segreto". She was a great success, and then worked in Munich in Germany for several years. After that, she tried to get work in the Opera in Sweden again, but her place there was occupied by a new singer, Henriette Widerberg.
The last years, she lived in Parma, where she died after she had fallen from a window.