[2] In 1994, Zimmer was appointed by the federal government to the Immigration and Refugee Board of Canada as its assistant deputy chairman heading the Toronto office, and was shortlisted for the national chairmanship in 1999.

In the 2003 Ontario provincial election, Zimmer ran as the Liberal candidate in the riding of Willowdale, defeating Progressive Conservative incumbent and sitting Minister of Municipal Affairs David Young by 1,866 votes.

Zimmer fended off a challenge by high profile local councillor David Shiner in 2007,[9] and was re-elected again 2011,[10] and 2014,[11] increasing his share of vote and margin of victory each time.

[15] Zimmer helmed the ministry, the second smallest in the government,[a] during a period indigenous issues increasingly became a front burner, mainstream area of political discourse, rapidly gaining awareness among the voting public and unprecedent attention from the Ontario government led by Wynne and the federal administration of Liberal Prime Minister Justin Trudeau.

Under Zimmer's watch, the indigenous affairs ministry was thrusted into roles with much greater prominence and sensitivities while substantially expanded its scope in many of the government's economic policy initiatives.

For most of his tenure, the ministry's most senior civil servant and its chief political advisor[b] were both long-time associates of Wynne.

Zimmer oversaw Ontario's Response to the Truth and Reconciliation Commission, including a $250 million special investment package, close to twice the ministry's annual budget, on an integrated suite of education, health, cultural and social services programs focused on reconciliation and addressing the legacy and negative impacts of residential schools.

As part of Ontario's response, in June 2016, his ministerial title was renamed to Minister of Indigenous Relations and Reconciliation,[16] a move later reversed when the Liberals were ousted from government.

He was determined to personally visit as many of Ontario's First Nation Communities as possible, including remote reserves in extremely poor conditions that are difficult to reach.

[c] His successor Greg Rickford at the height of his power was charged with leading natural resources, forestry, northern development, mines, and energy in addition to the indigenous ministry.