Prior to elected office, Johanson served as the Deputy Chief of Staff at the United States Mint, a Deputy Associate Director in the White House Office of Presidential Personnel, a Deputy Chief of Staff to Lieutenant Governor of HawaiiDuke Aiona, a legislative analyst for State RepresentativeLynn Finnegan and the Commissioner of the Hawaii Legislative Federal Economic Stimulus Program Oversight Commission. When Republican State Representative Lynn Finnegan of the 31st District retired to run for Lieutenant Governor of Hawaii in the 2010 election, Johanson ran to succeed her. He defeated Democrat Eleanor Lei Sharsh in the general election by 3,303 votes to 2,553. Shortly after being sworn in, he was elected by his Republican colleagues to serve as the House Minority Whip. Johanson was redistricted to the 32nd District for the 2012 election. He defeated Democrat Eleanor Lei Sharsh in a rematch by 4,017 votes to 2,143. In 2013, he was elected House Minority Leader. Widely regarded to be a "rising star" in the Hawaii Republican Party, at just 32 years of age, he was the youngest person to hold the post in recent history. He was re-elected in 2014, defeating Sharsh in a second rematch by 3,698 votes to 1,452. After the 2014 elections, all eight Republicans in the State House were unable to organise because some of them wanted to replace Johanson as Minority Leader, saying that he was "too collaborative" with the Democrats. On December 29, 2014, he changed his party affiliation and joined the Democratic Party. He said that he had been increasingly at odds with Republican Party members, who had become "more narrow in their demands for ideological purity as well as in their demand for a combative tone and posture" and that rather than "being focused on remaining the loyal opposition as some would like", he "look forward to making a collaborative, constructive and positive difference." Reaction to his decision in the Republican Party was mixed. State Representative Cynthia Thielen, the longest-serving Republican House member, cried when lamenting his move, blaming right-wing conservatives for pushing him out and saying that the Party had lost a "wonderful, moderate voice. And I'm very sorry about it." By contrast, Party Chairwoman Pat Saiki called his move "disgraceful" because he had been re-elected as a Republican less than two months prior, saying that it was "disappointing" that he was "willing to sacrifice his own political principals for his own ambition." Johanson replied that his decision had been a "gradual and deliberative one" over the last four years and said that many voters cast a ballot for him despite his being a Republican, not because of it. He was subsequently replaced by Beth Fukumoto as minority leader, who would later leave the party and join the Democrats herself for similar reasons in 2017. Johanson was re-elected to a fourth term as a Democrat in the 2016 elections, defeating Republican opponent Eric Ching by 4,170 votes to 1,787.
