Besides being councillor from 2017, he was also chairman of the municipality's work and welfare committee from 2015 to 2021.[2] As such, Rosengren become nationally known as a staunch supporter of the "Arbetslinjen", including cutting income tax, reducing social security payments and implementing measures to combat social exclusion.[3] He was named "Sweden's most powerful politician under the age of 30" in 2021.[4]
In his youth, he was district president of the Moderate Party Youth league in Kronoberg, and a member of the national board of the Youth league from 2014 to 2016.[5]