The company has no registered lobbyists yet, unlike competitors Google and Facebook, which each spend millions of dollars on lobbying every year. Carty's job will be to monitor legislation and educate lawmakers about Twitter, a company spokesman said.
Carty will register as a lobbyist if his job activities require him to, the spokesman added.
Twitter, a microblogging service with about 500 million users, was founded in 2006 and hired its first Washington D.C.-based employee, Adam Sharp, in 2010. Sharp helps politicians use Twitter to reach out to their constituents.
Although the company has mostly stayed out of public policy debates, it did come out against the Stop Online Piracy Act (SOPA), a controversial anti-piracy bill that sparked a massive Web protest earlier this year.
Twitter also could draw attention from policymakers over privacy issues as it, like Facebook and Google, tries to raise more revenue from ads.