Arment worked as lead developer and chief technology officer (CTO) of the Tumblr microblogging platform and social networking website from its inception in February 2007 until September 2010, when he left to concentrate fully on Instapaper, a tool for saving web pages to read later. Arment announced on April 25, 2013, that he had sold the controlling interest in Instapaper to Betaworks.

Between November 2010 and December 2012, Arment hosted a podcast, Build and Analyze, with Dan Benjamin on 5by5 Studios. More recently, he has hosted two podcasts, Neutral and Accidental Tech Podcast, with John Siracusa and Casey Liss. He also hosts Top Four with his wife Tiffany Arment and Under the Radar with David Smith, both on Relay FM.

In October 2012, Arment released The Magazine, an electronic, biweekly publication. In May 2013, one month after the sale of Instapaper, Arment announced he was selling The Magazine to Glenn Fleishman, its editor.

In July 2013, Marco released Bugshot; an application to quickly mark up screenshots so that beta testers of Overcast could easily send him bug reports. Marco sold Bugshot to Lickability and the app was renamed to Pinpoint.

In July 2014, Arment released Overcast, a podcast application for iOS. The application had been in development since the fall of 2012, and was first publicly announced at the XOXO Festival in October 2013. In 2014 Marco invested $50,000 into Gimlet Media.

On September 16, 2015, Arment released Peace, a Safari content blocker for iOS 9 using the Ghostery database. After Peace had held the top spot on the App Store’s list of paid apps for 36 hours, Arment pulled it from the App Store, stating he didn’t “feel good” with its resounding success. He elaborated, “While [ad blockers] do benefit a ton of people in major ways, they also hurt some, including many who don’t deserve the hit.”

His blog, which he began in December 2006, averages over 500,000 page views per month as of July 2014.