Federal NDP President

At the federal NDP convention of 2001, Giambrone won a first two-year term as president of the party. He was a grassroots challenger to the "official slate" candidate customarily supported by the party brass, but the official slate candidate stood down in Giambrone's favour. The president of the NDP is the administrative chairperson of the party, chairing party conventions, councils and executive meetings. Giambrone was 24, and no younger person had ever become president (or leader) of a major Canadian party. He became a frequent guest on television and figure in the news, and travelled the country extensively speaking to New Democrats.

Seeking re-election at the 2003 convention at which Jack Layton was elected party leader, Giambrone faced a strong challenge from respected New Brunswick NDP leader Elizabeth Weir. Midway through the convention, Giambrone and Weir decided to seek a co-presidency. Many delegates balked, especially at the assumption that they could push through a sudden constitutional change in a party often dearly concerned with internal process. The joint ticket was withdrawn, and Giambrone won a second term against Weir and a challenger from the NDP Socialist Caucus.

He announced in the summer of 2006 that he would not seek a third term as federal NDP president. Anne McGrath was elected to succeed him at the party convention in Quebec City.

TTC Chairman

As chair of the TTC, Giambrone oversaw a large expansion of bus service in Toronto, which saw then-record ridership,^[10] and the launch of a new, improved TTC website ^[11] electronic service advisories,^[12] as well as next vehicle arrival information systems,^[13] and an affinity discount program for the Metropass. Some of these ideas were discussed at Transit Camp, a community consultation early in his term as chair.

Giambrone also established the station modernization and station renaissance renewal programs for TTC subway stations, the latter in conjunction with the Toronto Community Foundation. Giambrone also started a program to install bicycle racks on all TTC buses.^[14] although naysayers complained that they were not worth the expense and seldom used.^[15]

Giambrone together with Mayor David Miller unveiled Transit City, a $10 billion expansion^[16] of light rail into neighborhoods and areas not served by rapid transit, and reaching into the GTA.^[1] Transit City calls for 120 kilometres of electric light rail along seven new routes.^[17] Premier Dalton McGuinty announced funding for Transit City as a component of its MoveOntario 2020 plan on June 15, 2007.^[18] Transit City was also included in Metrolinx's "Big Move" funding plan when it was released on November 27, 2008.^[19]

On April 24, 2009, Giambrone announced that the TTC had opted to replace its aging fleet of Canadian Light Rail Vehicles and Articulated Light Rail Vehicles with 204 new streetcars, procured competitively from Bombardier Transportation. New streetcars began to enter service in 2012. The deal, valued at more than $1.2 billion, called for the streetcars to be manufactured in Thunder Bay.^[20]

Giambrone announced the launch of the TTC's Transit City Bus Plan on August 21, 2009.^[21] The plan took the 'network approach' of the Transit City Light Rail Plan and applied it to buses, creating a network of 10-minute service on 21 of its 139 bus routes which began in the fall of 2010.

Giambrone appeared on CP24's monthly television show called On The Rocket in which he rides a streetcar and discusses daily transit operations, improvements, plans and concerns.

In the wake of a sex scandal (see 2010 mayoral campaign), fellow councilor Brian Ashton called for Giambrone to resign as chairman of the TTC, which had been beset by bad publicity. Ashton said "The TTC is under serious assault around issues of customer relations and performance. The union is now fragmented from the management. I think his moral authority has been totally debased and undermined," said Ashton. "I think mentally he's going to be distracted by both his mayoralty bid and this sex scandal. His ability to come to terms with the TTC issues is tremendously weakened."^[22] He did not step down, but also did not seek re-election and so his term as TTC chair ended in December, 2010.

2010 mayoral campaign

Giambrone launched a campaign for Mayor of Toronto in the 2010 municipal election to succeed David Miller on February 1, 2010,^[23] ending it on February 10, 2010.^[24] Before Giambrone entered the race, two polls showed him in second place among declared mayoral candidates,^[25] though he also had a high unfavorability rating of 40 per cent.^[25][26] He remained a leading candidate until February 9, when the Toronto Star published an interview with undergraduate student Kristen Lucas, who revealed that she had a past sexual relationship with Giambrone, including acts in his city office.^[27] Giambrone apologized for an "inappropriate relationship" and admitted to intimate relationships with women other than his live-in partner, to whom he is now married.^[28] In the immediate wake of the revelations, Giambrone initially pledged to continue with his mayoral campaign,^[27] but announced his withdrawal the following day.

Giambrone did not run for re-election to his council seat, and his executive assistant Kevin Beaulieu was defeated by Ana Bailão.^[29] Giambrone claimed French lessons as a city council expense, as he was Toronto's representative to the Francophone Association of Municipalities of Ontario and does interviews with francophone media.^[30]

Scarborough—Guildwood provincial by-election

On July 5, 2013, Giambrone resigned as co-chair of the candidate search committee and announced his candidacy for the Ontario New Democratic Party's nomination for the August 1, 2013 provincial by-election in Scarborough—Guildwood; he won the nomination on July 7 over community activist Amarjeet Chhabra, reportedly by a margin of 18 to 14.^[31][32]

Giambrone's nomination was the subject of a threatened legal challenge by Chhabra who alleged that 12 of the 32 individuals who voted at the nomination meeting were not on party membership lists and may not have been entitled to vote according to party rules that require an individual to be a party member for 30 days and to live in the riding in order to be able to cast a ballot. Chhabra said she would not proceed with her attempt to force a new nomination meeting due to the tight time frame before the election.^[32]

Giambrone came in third place with 28 per cent of the vote, an improvement of 9 per cent from the previous NDP candidate in the 2011 provincial election.