After receiving her law degree, Jaatteenmaki held several civil and government offices. She has been a member of the Fundamental Rights Committee since 1991; a member of the Bank Committee; member of the Tiitinen Constitutional Executive Committee; and member of the 2000 Constitution Committee. In 2000 she became the deputy chair of the opposition Center Party under the previous prime minister, Paavo Lipponen of the Social Democrats; in 2002, she ascended to chair of the Center Party.
In March 2003, the contest between Jaatteenmaki's Center Party and Lipponen's Social Democrats was considered a virtual dead heat until the final votes were tallied. he election was very close, with the Center Party and the Social Democrats alternatively in the lead as the votes were tallied. In the end, the Center Party prevailed by a narrow margin, capturing 55 out of 200 seats in the parliamentary election. News agencies reported that Jaatteenmaki's campaign expenditures (at €70,689) far surpassed those of Lipponen (at €42,505). Although the two parties formed a coalition government, Lipponen said he would not be part of Jaatteenmaki's cabinet.