After practicing as a tax lawyer in New York City, Kay moved to Toronto, where, in 1998, he became a founding member of the National Post editorial board.

[6] Kay joined the National Post at its inception, in 1998, as a member of its editorial board, subsequently becoming the newspaper's Comment editor as well as a columnist.

In May 2011, HarperCollins published Kay's second book, Among the Truthers: A Journey Through America's Growing Conspiracist Underground (ISBN 978-0-06-200481-9).

[8] His participation in the project was criticized by conservatives in social media as well as by Sun News Network personality Ezra Levant, on whose 2009 book Shakedown Kay also worked as an editorial assistant.

[19] Since May 2018, Kay also hosted Quillette's Wrongspeak podcast, along with Debra W. Soh[20] until she quit at the end of first series of episodes (2018).

[23] He resigned as editor-in-chief of The Walrus on May 13, 2017, following a controversy around cultural appropriation in which Kay argued that concerns by Indigenous writers about the practice should be balanced against the right to free artistic representation.

Kay added that there had been no conflict between himself and the publisher of The Walrus and that he had been given a free hand to edit the magazine and its website and that the pressure he had felt to self-censor was in relation to his non-Walrus work.