Williams won election Saturday, becoming the first D.C. mayor to assume the top post of such a group. The league has more than 18,000 cities and towns among its members. Williams will be the league's chief lobbyist on Capitol Hill.
But he hopes to get something out of it for D.C. too.
Williams is calling on his colleagues to advocate for full-voting rights for the nation's capital. He used a few minutes of his inaugural speech Saturday to ask for their help.
Williams is promising an agenda focused on homeland security, education, public safety and affordable housing.
The new post will take the mayor away from D.C. on league travel. The Washington Post reports that he's been gone for nearly a quarter of his second term already.