Hemmer has covered many major stories since his tenure at the network, including: the British sailors' hostage situation in Iran (where he reported live from London); the crisis in the Middle East; the rescue efforts for miners trapped in West Virginia; and the recovery efforts in Baton Rouge after the devastation of Hurricane Katrina. Additionally, he has provided live coverage while embedded with U.S. Marines in Camp Fallujah, Iraq. And, most recently, Hemmer has provided coverage of the 2008 presidential election from various locations.

Prior to FNC, Hemmer worked as an anchor and correspondent at CNN where he co-hosted "American Morning." While there, he also anchored "CNN Live Today" and "CNN Tonight." He provided live coverage from Rome and Vatican City on the death of Pope John Paul II and in 2004 he covered both the Democratic and Republican conventions and served as one of CNN's prime-time anchors on election night.

In 2003, Hemmer traveled to Kuwait to report on escalating tensions in Iraq and remained on-site to cover Operation Iraqi Freedom when the war began. Prior to that, he reported from Ground Zero in the aftermath of the 9/11 terrorist attacks and, shortly after, traveled to Jerusalem to provide live coverage on tensions in the Middle East. Hemmer also reported live from Tallahassee, Florida for 37 straight days during the 2000 election recount.