Bjørnø was a member of Porsgrunn city council from 2003 to 2011. He was elected to Telemark county council in 2011 where he served as deputy mayor.[1]
In the 2013 Norwegian parliamentary election, he was nominated in the third spot on the Telemark Labour ballot. He fought for the ballot position against Labour Youth leader Eskil Pedersen and won with 74 votes against 55 votes for Pedersen at the Telemark Labour Party's nomination meeting in November 2012.[2] Eskildsen had the support of Skien Labour party and Telemark Labour Youth, while Bjørnø was supported by Porsgrunn Labour Party and several other district branches where a major argument in favour of Bjørnø was that he was situated in Telemark as opposed to Pedersen who had lived in Oslo for a long time.[2]
In the general election, Labour Party got three representants from Telemark and Bjørø became representative number 5 from Telemark.[3]