He was a member of Provincial Parliament (MPP) from 1971 to 1987, and was a Cabinet minister in the Progressive Conservative governments of Bill Davis and Frank Miller.
His supporters included Keith Norton, Leo Bernier, Margaret Birch, Robert Eaton, Gordon Dean, Bob Welch and Norm Sterling.
Lou Parsons, a senior Miller adviser, later acknowledged, "We wouldn't have won it against Dennis [...] Our winning strategy was always to be against Larry ... and in the end we were lucky.
In this leadership race, Timbrell announced he would not support the full funding of Catholic schools (which had previously been agreed to by all parties in the legislature) unless amendments were put forward guaranteeing entry to non-Catholic teachers and students.
Norm Sterling, an inveterate opponent of Catholic school funding, derided Timbrell's position as opportunistic and crossed over to Grossman.
[15] Despite no longer being an MPP, Timbrell was seen by many as the front runner to succeed Grossman in the 1990 Ontario PC Leadership Race, but he chose not to run, instead supporting the candidacy of Dianne Cunningham, who would eventually lose to Mike Harris.
His work with the leadership of these hospitals resulted in renewed/revised mandates which emphasized the unique attributes of each institution while ensuring minimal overlap and with co-ordination of their respective areas of expertise.
In Ottawa Timbrell oversaw the recruitment of a complete new Board of Governors, drawn from the vast region served by TOH; having dismissed the incumbent CEO of the Hospital, he appointed a new President from the ranks of the Senior Team and created the position of Vice-President and Chief Operating Officer.
In his earlier role as President of the OHA, he established a very productive relationship with NDP Health Minister (now Senator) Frances Lankin which contributed to significant improvements in Ministry programs and policies directly affecting all hospitals in Ontario.