[1] Mathilde Nielsen was born in Bergen,[2] the daughter of the journeyman Niels Christian Nielsen and Marte Elisabeth Olsen (Larsen).
She married Gunnar Olsen Alvorn in 1891.
[4] Mathilde Nielsen was a pupil of the Danish theater director Frederik Adolph Cetti in her hometown of Bergen.
She followed Cetti in the 1870–1871 season in Bergen and Trondheim.
[3] In the 1880s, Nielsen was a member of the theater association of the East Kristiania Labor Union (Norwegian: Kristiania østre Arbeidersamfund).