Andrea Horwath

Later in 1997, she was elected to Hamilton City Council for Ward Two, outpolling two incumbents who had represented the area for more than 20 years. She emerged as a prominent voice for the political left in the city, and was re-elected to council in 2000 and 2003. During her three terms as city councillor, she chaired the solid-waste-management committee and the municipal non-profit housing corporation.

Horwath was elected to the Legislative Assembly of Ontario in a 2004 by-election in the then-extant provincial riding of Hamilton East, defeating Liberal candidate Ralph Agostino to succeed the deceased Liberal member Dominic Agostino, Ralph's brother. 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 2007 election, Horwath ran in the new riding of Hamilton Centre, due to redistricting that divided her former Hamilton East riding between Hamilton Centre and the new riding of Hamilton East—Stoney Creek. Horwath's new Hamilton Centre riding included approximately half of her former riding as well as a portion of the former Hamilton West riding where she had run federally in 1997. It also included her entire former city council ward.

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.

Horwath during a debate in the 2009 NDP leadership election

On November 7, 2008, Horwath officially launched her campaign to win the party's leadership. The leadership election was held March 6–8, 2009. Horwath led on the first two ballots, and won on the third ballot with 60.4% of the vote defeating Peter Tabuns, Gilles Bisson and Michael Prue.[10]

The 2011 provincial election saw a rise in support for the NDP under Horwath's leadership. The party won more than 20% of the popular vote for the first time since 1995 and almost doubled its seats to elect 17 members of the legislature. The election also resulted in the Liberal government of Dalton McGuinty being reduced to a minority government with the NDP holding the balance of power.

In April 2012, Horwath passed a leadership review at the party's convention with 76% support.

Horwath during the 2014 provincial election campaign

In the 2014 provincial election, the NDP was able to maintain its seat count of 21 at dissolution despite the loss of three seats in Toronto, but lost the balance of power when the Liberals took a majority win in the election. Horwath has faced criticism from some party members and progressives for running a populist campaign which they described as right-wing.[11] 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.[12]

Horwath ran in her third election as NDP leader against the Liberal government led by Kathleen Wynne and a Progressive Conservative Party led by Doug Ford. Horwath promised to introduce "Canada's first universal Pharmacare plan", highlighted by a universal dental plan and a prescription drug plan that "will initially cover 125 of the most commonly prescribed drugs".[13][14] She also promised a child care plan in which seventy per cent of Ontario parents "would either have free child care or pay an average of $12 a day in a licensed not-for-profit daycare".[14] Horwath promised to return Hydro One to public ownership by buying back privately held shares.[15] She also said that she would close the Pickering Nuclear Generating Station immediately, while the other party leaders have pledged to keep it open until 2024.[16] The NDP promised to increase corporate tax rates from 11.5 to 12.5 per cent,[17] as well as introducing an income tax increase for those earning over $220,000 per year.[18] Horwath said the province would fund half of the operating cost of municipal transit[19] and indicated that she would not introduce back-to-work legislation.[15] The party's support in public opinion polls increased in May 2018,[20] leading to greater media attention and greater scrutiny. With her party gaining official opposition status, she became the Leader of the Official Opposition during the 42nd Parliament, the second highest number of seats in the party's history.[21] The NDP took all of old Toronto (i.e., what was the city of Toronto before the 1999 creation of the "megacity" of Toronto), as well as all but one seat in Hamilton and all but one seat in Niagara.

Horwath and the NDP released their 2022 platform in April 2022.[22] Horwath was re-elected in Hamilton Centre, but the NDP lost 7 seats.[23] Horwath resigned as leader election night.

Horwath took office as mayor on November 15, 2022, becoming the first woman to serve as mayor of the city of Hamilton. Prior to amalgamation the suburbs of Stoney Creek and Ancaster each had had women mayors. The first regional chair for the Region of Hamilton-Wentworth was also a woman. In April 2023, Hamilton city council declared a state of emergency over opioids, homelessness, and mental health.[27]