Born in Hyderabad, India, the oldest of six children, to a construction company proprietor, Siddiqui enjoyed what he characterizes as a secure and affectionate, "middle- to upper-class" childhood. At Osmania University in Hyderabad he earned degrees in science and journalism. In 1963, he joined the Press Trust of India as a reporter and copy editor. When his father fell ill, Siddiqui briefly left journalism to manage his father's company, which he did until 1967. While at the Press Trust he met Roland Michener, then Canada's High Commissioner to India who encouraged him to emigrate to Canada. By 1968 he had taken a job at the Brandon Sun in Brandon, Manitoba, reporting on municipal and provincial politics from 1968 to 1978. In 1978, he joined the Toronto Star, becoming foreign affairs analyst in 1979, news editor in 1982 and national editor in 1985. From 1985 to 1990, Siddiqui was National Editor, responsible for coverage of federal and provincial affairs. From 1990 to 1998, Siddiqui was the Star's editorial page editor, and on his departure from that position, he was given the title of 'editor emeritus' and a twice-weekly column, which focused on national and international politics as well as cultural and Muslim issues. Siddiqui has written from a left-of-centre perspective on such issues as: the war in Iraq and terrorism. During his tenure The Star advocated a distinct constitutional status for Quebec and protection of French minorities outside Quebec. He is the past president of PEN Canada and chair of International PEN’s Writers-in-Exile Network. He is on the board of directors of the Calmeadow Foundation, the Canadian Club of Toronto, and the advisory board of the Ryerson University School of Journalism. Siddiqui has authored Being Muslim; edited An English Anthology of Modern Urdu Poetry ; assisted in Christopher Ondaatje’s Sindh Revisited, following the footsteps of Victorian explorer Sir Richard Burton; and contributed to Canada and Sept. 11, published by the University of Calgary and Drawing Fire: The State of Political Cartooning, a colloquium of North America’s top cartoonists and editors, at the American Press Institute. Siddiqui continued as a columnist at the Star until 2015. His farewell column of 1 April 2015 marked his retirement from journalism.
Writing in Toronto Life in June 2001, Robert Fulford maintained that "Siddiqui makes the most strenuous effort to bathe Third World countries in a soft light. No matter how outrageous its actions, a non-Western government can usually count on him for a little understanding." In a June 2013 column, Siddiqui demanded that Canada's Minister of Citizenship and ImmigrationJason Kenney "should resign or be fired." In an article the following month, Siddiqui accused Kenney, of "turn immigration into a tool of anti-Arab and anti-Muslim bigotry," and of "barring refugee claimants from ‘safe third countries.’" Chris Selley, writing in the National Post, pointed out that immigration levels of individuals speaking Arabic as a first language have actually increased during Kenney's tenure. Regarding Siddiqui's second claim, Shelley argued that refugees from "safe third countries" are not automatically refused refugee status by Canada but rather are "directed them into an expedited system with a somewhat weaker appeals process." Selley also argued that Siddiqui's accusation against Kenney in this regard was "a massive factual error that you’d think an expert on this matter wouldn’t make." In a May 2016 Toronto Sun column, Tarek Fatah claimed Siddiqui "fanned the flames of victimhood among Muslims, while looking the other way as Islamism spread". Fatah provided several quotes of Siddiqui that suggest he is supportive of sharia law and the governments of Saudi Arabia and Iran. Siddiqui also allegedly "mocked a Quebec Muslim legislator, Fatima Houda-Pepin, who led the charge against creeping sharia".