He is a socialist.

In 2006, McDonnell said that "Marx, Lenin and Trotsky" were his "most significant" intellectual influences.

McDonnell has explored ideas surrounding "alternative models of ownership", publishing a report on the subject in June 2017 and hosting a discussion conference in London in February 2018. The report sets out the "practicality and necessity of a shift to a variety of alternative forms of ownership and control of productive enterprises, including co-operatives, municipal and locally-led ownership forms, and...new democratic forms of national ownership".