A veteran federal politician who began his political career in 2000, McCallum has served in the governments of Liberal prime ministers Jean Chrétien, Paul Martin, and Justin Trudeau.

One of his most influential academic contributions was an article in the American Economic Review,[6] which introduced the concept of the home bias in trade puzzle.

He consistently achieved the highest media coverage of bank chief economists, making regular appearances on CBC's The National as an economics panellist.

[9] In July 2020, McCallum's employment by the Wailian Group, a Chinese company that assists with immigration to Canada, sparked calls from oppositions MPs and Democracy Watch for McCullum to be investigated by the Ethics Commissioner for potential breaches of the Conflict of Interest Act.

[14] He also retroactively reversed an inequity which awarded up to $250,000 to military personnel who lost their eyesight or a limb while on active service - but only to those with the rank of colonel or above.

In November 2002, while still serving as Defence Minister, McCallum encountered further controversy when officials refused to allow him to board an Air Canada flight because his breath smelt heavily of alcohol.

[19] In January 2003, McCallum suggested Canadian troops could avoid so-called "friendly fire" incidents by wearing some of female Conservative MP Elsie Wayne's clothes.

[20] Under Paul Martin, McCallum introduced a new charter for younger, postwar veterans who have been physically or mentally injured while serving in the Canadian Forces.

[22] On 4 November 2015, he was appointed the Minister of Immigration, Refugees and Citizenship in the newly elected 29th Canadian Ministry of Justin Trudeau.

[26] On 23 January 2019, McCallum spoke to Chinese-language Canadian and state-owned Chinese media in Markham, Ontario,[27] concerning the detention and extradition request by the United States, which resulted in the arrest of Huawei deputy chairwoman Meng Wanzhou,[28] who was awaiting court judgement.

The Canadian Federal government reaffirmed that it is obliged to follow judicial protocol and that the arrest was not political in nature.

McCallum shared his thoughts with the media by restating public facts that could make Meng's legal defence case strong against this extradition request.

[30] That week, McCallum was further quoted as later telling a Toronto Star journalist it "would be great for Canada" if the US extradition request were dropped, conditional on release of Canadians since detained in China.

[31] On 26 January 2019, McCallum submitted his resignation as ambassador to China, at the request of Prime Minister Justin Trudeau, who did not disclose the reasoning behind this decision.