Allison started her career in 2006 as a Lacrosse Coach in Westonbirt School at English Lacrosse Association. In 2007-2008, she was an Assistant Lacrosse Coach at Longwood University. In 2008 she became an Editorial Assistant at The Ripon Forum. In 2009-2010, she served as a Staff Assistant at Distilled Spirits Council of the United States. During 2011, Allison worked as a Production assistant at Roll Call and Current Correspondent at The Current Newspapers. 
In 2012 Allison joined CNN as a CNNPolitics.com Intern. She quit the company in 2018. Currently, Brennan is a Senior producer of All Around Project and Reporter at Belgrade News. In addition, she is a founder and multimedia journalist at The Brave Productions.