Gardner began her career on Canadian Broadcasting Corporation radio in 1990 on the program As It Happens, and worked for more than a decade as a producer, reporter and documentary maker for CBC Radiocurrent affairs and for Newsworld International, focusing on pop culture and social issues. In March 2006, she succeeded Claude Galipeau as senior director of the CBC website and Internet platform, CBC.ca, building its staff from 35 to 160.
Wikimedia
In May 2007, Gardner resigned from CBC, and shortly thereafter began consulting for the Wikimedia Foundation as a special advisor on operations and governance. In December 2007, she was hired as the foundation's executive director. Over the next two years, she oversaw growth of the staff including the addition of a fundraising team, and a move of the headquarters from St. Petersburg, Florida, to San Francisco, California. In October 2009, Gardner was named by The Huffington Post as one of ten "media game changers of the year" for the impact on new media of her work for Wikimedia. One of the issues that Gardner addressed while she was a Wikimedia Foundation executive director was gender bias on Wikipedia. She listed nine reasons why women don't edit Wikipedia, culled from comments by female Wikipedia editors:
Fewer opportunities than other sites for social relationships and a welcoming tone
On March 27, 2013, Gardner announced she would be leaving her position at the Wikimedia Foundation. She stated that the Wikimedia Foundation is doing well now but that the Internet is not, and that she planned to help in that area going forward. Gardner identified the "turning point" for her decision to move on as her involvement in the 2012 Wikipedia blackout protesting the Stop Online Piracy Act and the Protect Intellectual Property Act, protests that "started me thinking about the shape the Internet was taking and what role I could play in that." In 2013, Gardner received an honorary doctorate from Ryerson University, her alma mater. It was announced on May 1, 2014 that Lila Tretikov would be replacing Gardner, and would take over as executive director of the Wikimedia Foundation on June 1, 2014.
In September 2018, Gardner co-founded The Markup with Julia Angwin and Jeff Larson as a continuation of their work at ProPublica. With $20 million of initial funding from Craig Newmark, the site would work to cover news about "Big Tech" and its impact on the public. Initially, Gardner was set to serve as executive director, Angwin as editor-in-chief, and Larson as managing editor, with a launch date of early 2019. In April 2019, Gardner fired Angwin over creative and managerial differences. Larson was named editor-in-chief. Five of seven journalists on the staff resigned. In a letter to Newmark, Angwin said Gardner wanted to turn The Markup into a “cause” rather than a “publication.” Angwin also said Gardner ranked reporters in job interviews according to how negative they viewed tech companies and suggested using headlines like "Facebook is a dumpster fire." Gardner responded that The Markup's mission had not changed. Gardner and Larson left The Markup the following month, and Angwin was reinstated as the website's editor-in-chief in August 2019.