Armstrong coined the term netroots, and was referred to as "The Blogfather" for having mentored many other famous bloggers such as Markos Moulitsas in their early years.

[7][9] In 2001, he founded MyDD, a blog which covered American politics, in which Armstrong's work was described by Salon as being "fiercely partisan but not radically left-wing.

In 2008, London mayoral candidate Brian Paddick, a UK Liberal Democrat, brought aboard Armstrong[5] "to help boost his campaign's online presence".

[19][20] In 2006, Armstrong and Markos Moulitsas of Daily Kos co-authored the book Crashing the Gate: Grassroots, Netroots, and the Rise of People Powered Politics.

[21] Armstrong, along with Markos Moulitsas and Tyler Bleszinski, founded the Washington, DC–based Vox Media, a network of blogs and online verticals, with funding led by Accel Partners.