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. 