Shirley Krug was born in Milwaukee in 1958. Krug was initially elected to the 15th Assembly District in 1984 but this became the 12th Assembly District after the district was redrawn. In a 1998 three-way race for the minority leader position, Krug defeated Jim Kreuser (D-Kenosha) and Marlin Schneider (D-Wisconsin Rapids) to become the first woman to serve as the Democratic Assembly Minority Leader (1998-2002).
Krug withstood a scandal concerning the shakedown and intimidation of lobbyists, in the late 1990s. Several lobbyists testified that they had been intimidated into and/or traded campaign contributions for favorable consideration on pending legislation. While Krug was mentioned in several testimonies, no evidence ever directly implicated her. Moreover, the charges seemed to be politically motivated as members of both the Democratic and Republican parties were implicated in the scandal. Nevertheless, this scandal indicated general campaign finance irregularities occurring in both the Democratic and Republican Parties in the state of Wisconsin during the late 1990s.
Krug was replaced as Minority Leader by Spencer Black (D-Madison) after the 2002 election. In that election, Democrats lost two seats giving the Republicans a 56-43 majority in the Assembly. Citing the losses and controversial leadership decisions, some members of the Democratic Assembly delegation decided that new leadership was needed to counter the growing dominance of the Republican Party in Wisconsin politics. The removal of Krug from her leadership position was criticized by some Democratic members because it happened with little warning and just days after the death of her mother. The timing of the ouster was more troubling to some of the Democratic delegation than the actual replacement of Krug as their leader.
Krug made an unsuccessful bid for Wisconsin's 4th Congressional District seat during the 2003-2004 election cycle. During the primary, Krug dropped out of the congressional race after she failed to secure any key endorsements, especially from women's groups. State Senator Gwen Moore (D-Milwaukee) eventually succeeded in winning the Congressional seat.
In 2004, Krug did not seek re-election to the Wisconsin State Assembly, ending her term of office in January 2005.
