It's the first time a woman has won the prize on her own.
Guðnadóttir won the prize at the industry awards ceremony last night with her score for Joker. She thanked both the director Todd Phillips and lead actor Joaquin Phoenix in her acceptance speech, saying, "Thank you, Todd, for inviting me on the journey of a lifetime, for all the trust and your faith and your openness. Thank you, Joaquin, for making my job really easy with a spectacular, unbelievable performance."
Guðnadóttir is the first woman to win the award outright since the Golden Globes was launched in 1947. Lisa Gerrard won the prize with Han Zimmer with Gladiator in 2000.
Guðnadóttir has released several albums on Touch and has worked with the likes of Pan Sonic, The Knife, Sunn O))) and Jóhann Jóhannsson. She's also written scores for Chernobyl and Sicario: Day of the Soldado.
Listen to "Subway" from Joker.
Photo credit: Kevin Winter