[7] She is named after her father Andrew, an ethnic Hungarian who had immigrated to Canada from Slovakia, and worked on the assembly line at the Ford Motor Company plant in Oakville, Ontario.

[8] In the Canadian federal election of 1997, she was the NDP candidate against incumbent Liberal Stan Keyes in the riding of Hamilton West.

Although unsuccessful, her second-place finish was a significant improvement on previous NDP efforts in the riding, and gave her an increased level of prominence in the city.

Winning 63.6 per cent of the vote, up from the NDP's 29.4 per cent in that riding six months earlier, her landslide victory boosted the NDP's seat count over the threshold for official party status in the legislature, and helped give the federal New Democratic Party a bounce in Hamilton that would continue into the federal election shortly thereafter.

In the lead up to the campaign, Horwath was expected to face Hamilton West Liberal incumbent Judy Marsales.

However, Marsales opted not to run for another term, and Horwath easily defeated Liberal candidate Steve Ruddick on election day.

[12] Following this, Premier Kathleen Wynne formally asked Lieutenant Governor David Onley to dissolve the legislature and call an election for June 12, 2014.

[14] Despite criticism of her leadership from some quarters, Horwath received a slightly increased level of support, 77%, at the party's post-election convention held on November 15.

[21] Horwath said the province would fund half of the operating cost of municipal transit[22] and indicated that she would not introduce back-to-work legislation.

The NDP's campaign focused on increased funding for social programs and government services, which would be paid for through higher taxes on businesses and individuals earning over $200,000 per year.

The NDP also proposed to expand COVID-19 vaccine mandates, implement a mixed member proportional electoral system, to close down all privately owned long-term care facilities and to stop the construction of new highway projects.

[36][37] Hamilton’s ethics watchdog later cleared Horwath of wrongdoing in response to a complaint over her role in advancing the plan.

[39] In August 2024, she endorsed a previously proposed plan on sanctioned encampments offering tiny homes to unhoused people.

[40] The city later banned tents in public parks, and was sued by 14 unhoused residents, citing alleged infringed charter rights.

[45] In a March 2011 interview with the Toronto Star, she spoke publicly for the first time about the breakup of her longtime relationship with Julian's father, Hamilton businessman Ben Leonetti.

[47] In March 2012, Horwath received the EVE award which is sponsored by Equal Voice, a non-profit organization focused on promoting women in politics.