Residing in Chicago, Mosteller assisted with the Obama presidential campaign at its inception in February 2007. Working in Iowa, New Hampshire, and other pivotal states, he developed many of the policies and procedures used by the teams organizing the candidate's travels and events. He was involved in the on-site organization of the 2008 primary debates and executed the senator's role at the Democratic National Convention in Denver. Following Obama's election, Mosteller moved to Washington, D.C. and started in the Oval Office hours after the inaugural. Mosteller was one of the longest serving staff members in the administration. Anonymous colleagues noted that "if you [want a meeting] at two in the morning, you go through him." In a 2017 interview, Obama said of Mosteller, "He’s a tireless manager and leader with a knack for sweating the small stuff." In "West Winging It", a memoir published in 2018, the author references him as "responsible for maintaining dignity throughout the West Wing."