A pacifist environmentalist and expert on Icelandic crime thrillers emerged on Thursday as Iceland’s new prime minister, its fourth in two years, after three parties signed a coalition agreement.

Katrin Jakobsdottir, 41, chairwoman of the Left-Green Movement, will lead the government of the North Atlantic island of 340,000 residents after elections in October that were blighted by scandal and voter mistrust. (The job of president, held by the historian Gudni Johannesson, is considered a largely ceremonial role.)

Ms. Jakobsdottir will govern in coalition with parties of very different creeds: the conservative Independence Party and the center-right Progressive Party.

“It is important that we try to change the way we work together,” she said on Thursday at a news conference in Reykjavik, the capital, to announce the coalition. “This agreement strikes a new chord.”