From 2009 to 2011, Turnbull chaired the Maryland Democratic Party and led the Maryland Democrats to a reelection victories for incumbent governor Martin O'Malley and Senator Barbara Mikulski.

During her tenure at the DNC, Turnbull was a regular political commentator on MSNBC and Fox News from 1998 to 2010, as well as a guest on over 500 radio shows.

She was a principal speaker at events in more than 35 states and several foreign countries on behalf of the DNC and three presidential campaigns.

She previously served as the chair of the Montgomery County Board of Zoning Appeals.

On November 29, 2017, Ben Jealous, candidate for the Democratic nomination for Maryland governor in the 2018 election, announced the selection of Turnbull as his running mate.