Witmer began her political career as a school trustee, serving on the Waterloo County Board of Education from 1980 to 1990; she became its chair in 1984.

She was successful this time, defeating New Democrat Hugh Miller and Liberal Andrew Telegdi (later a federal Member of Parliament) [2] to become the first female MPP to be elected in the region of Waterloo.

[6] Harris's government was initially regarded by many as uniformly right-wing, although moderate Red Tory figures such as Witmer and Isabel Bassett eventually emerged in key portfolios.

[10] She ran in the 2002 PC leadership election to succeed Harris as Tory leader and Premier, but placed fourth on the first ballot and threw her support to the eventual winner, Ernie Eves.

Witmer considered running to succeed Eves in the 2004 PC leadership election, but ultimately supported John Tory's successful candidacy instead.

Witmer won the contest, ahead of political activist Georgina Bencsik and federal Member of Parliament Olivia Chow.

[14] On April 27, 2012 Witmer announced that she was resigning as an MPP, just seven months after the last election, and had accepted an appointment to head the Workplace Safety & Insurance Board.

One of Mrs. Witmer’s most notable accomplishments as Chair is the elimination of the unfunded liability (UFL) in 2018, almost a decade ahead of the legislated timeline of 2027.