Until her retirement from elective office in 2012 she was Iceland's longest-serving member of Parliament. In 1994, when she lost a bid to head the Social Democratic Party, she raised her fist and declared "Minn tími mun koma!" ("My time will come!"), a phrase that became a popular Icelandic expression. When she became Prime Minister in 2009, Forbes listed her among the 100 Most Powerful Women in the world.
In 2009, Jóhanna was proposed as Prime Minister because of her popularity among the general public and her good relations with the Left-Green Movement. An opinion poll by Capacent Gallup in December 2008 found 73 percent approval of her actions as a minister, more than any other member of the Cabinet.
Jóhanna married a man in 1970 and the couple had two sons. After their divorce in 1987, she joined in a civil union with Jónína Leósdóttir, an author and playwright, in 2002. In 2010, when same-sex marriage was legalized in Iceland, Jóhanna and Jónína changed their civil union into a marriage, thus becoming one of the first same-sex married couples in Iceland.
Jóhanna was the first of five openly LGBT heads of state so far. They include the former prime minister of Belgium, Elio Di Rupo, and the current heads of Luxenbourg, Ireland, and Serbia.