Among Nilssen's early novels are Innestengt i udyr from 1998 and Vi har så korte armar from 2002. In 2004 she published the children’s book Ronnys rumpe, and in 2005 the essays collection Hybrideleg sjølvgransking.[1]
Her theatrical debut was the play Skyfri himmel, which was staged at Rogaland Teater in 2006. She wrote the book Nesten frelst av Sigvart Dagsland in 2009, and the interview book Kjøkkenbenkrealisme. Ærlege historier om tidsklemma in 2012. She wrote the play Stort og stygt for the centennial of Det Norske Teatret in 2013.[1]
Her novel Tung tids tale from 2017 earned her the Brage Prize,[2] as well as the Nynorsk Literature Prize.[3] The novel describes how it is like to be the mother of a child with disabilities.[1] She was awarded the Dobloug Prize in 2019.[4][5] She wrote the satirical book Ikkje tenk på det in 2019, and the humoristic novel Yt etter evne, få etter behov in 2020.[1]
Nilssen has contributed to the literary magazine Vagant. She co-founded the magazine Kraftsentrum in 2005, along with Gunnhild Øyehaug, and was co-editor of the magazine until 2008. She was a member of Norsk kulturråd from 2010 to 2012.[1]