In 2003, Mark Fletcher started Bloglines, a free web-based news aggregation service. Using Bloglines, users can search, subscribe to, share and publish blogs and RSS feeds. Bloglines was named one of Time Magazine's Top 50 Web Sites for 2004, and was named the Best Blog/Feed Search Engine by the Search Engine Watch Awards in 2005. In February, 2005, Bloglines was acquired by Ask Jeeves, where Mark served as VP & General Manager of Bloglines until May, 2006.

In 1997, Mark started ONElist, a free Internet email list service. To that point, email lists had been difficult to set up and administer. Through ONElist, Mark set out to make email lists available to even novice users. As CEO, Mark raised money from CMGI and Bertelsmann Ventures in 1998. The service was the category leader from the beginning and in November 1999, ONElist acquired eGroups, its main competition. Yahoo acquired the resulting company, renamed eGroups, in June 2000 and the service is now called Yahoo Groups. At acquisition, eGroups served twenty million active users, one million email lists, and sent out over two billion email messages a month, making it one of the largest services on the Internet. Mark served as CEO of ONElist from inception until October 1999 and was CTO until the acquisition by Yahoo.

Prior to ONElist, Mark was a Senior Software Engineer for Sun Microsystems, where he worked on web enabled set top boxes. He came to Sun through the acquisition of Diba, a Menlo Park, CA startup working to develop an embedded web surfing software and hardware system. At Diba, Mark developed key embedded web browser technologies.