Lagesen was born in Bagn in the Valdres district.^[2] She started studying in 1933 with Mary Barratt Due,^[1][2] Nils Larsen,^[1][2] and Erling Westher^[2] in Oslo, and she debuted as a pianist^[3] at the University of Oslo's Aula Hall in 1935^[1][2] with the Oslo Philharmonic. After further study in Brussels and London, in 1938 she settled in Larvik, working for the Treschow family at Fritzøehus.^[2] After the Second World War, she established the town's Cecilia Society in 1945 and served as the head of NRK's junior orchestra from 1947 to 1951.^[1][3] Lagesen was often featured by NRK as a soloist with the Norwegian Radio Orchestra and she also appeared on TV abroad. She distinguished herself in 1949 as Norway's first female orchestra conductor.^[4][5] She was known for conducting a number of large-scale works, including Handel's Messiah^[3] and Bach's passions. In 1977 she conducted Bach's Christmas Oratorio from Kongsberg Church in three programs on NRK Television.

After studying in Paris with Nadia Boulanger^[1][2][3] and Eugène Bigot^[2][6] from 1955 to 1957, with classmates Per Nørgård and Mikis Theodorakis,^[2] she took a conducting exam and became a music teacher at Eik Normal School.^[2] She established the Friends of Music Society in Tønsberg and Larvik, as well as the Larvik Boys' Choir and Music Society. She conducted the royal-sponsored Robert Riefling seminar in 1968 and 1969.^[2] As a conductor, she led the Oslo Philharmonic, Bergen Philharmonic Orchestra, Norwegian Radio Orchestra, and Trondheim Chamber Orchestra. Lagesen was on the board of the Norwegian National Academy of Opera.^[7] As late as 1993, she lectured at the Norwegian Academy of Music on Edvard Grieg and his work for piano students.

Lagesen died at age 90 in Larvik.^[2][8]