Following the example of her elder sister, she was enrolled in the Royal Dramatic Training Academy in 1805 under the tutelage of Sofia Lovisa Gråå.
Justina Casagli was inducted to the Royal Swedish Academy of Music in 1817, the same year as her sister and Anna Sofia Sevelin, both of them elite singers of the Opera.
In 1818, Casagli left Sweden with her spouse and made her a successful Italian debut in La Cenerentola by Rossini in Turin.
She continued to tour Italy successfully the following years, and became the first Swedish opera singer to have made an international career.
In Lucca in 1827, she made another performance in Cimarosa's Il matrimonio segreto, which attracted fame, before finding permanent employment.