He attended Onondaga Junior-Senior High School in Nedrow, a suburb of Syracuse, New York. He then attended Ithaca College in Ithaca, New York, where he graduated magna cum laude with a Bachelor of Arts degree in 1995. Following graduation, he attended the Institute on Political Journalism at The Fund for American Studies, affiliated with Georgetown University in Washington, D.C.. He also studied at the University of Salamanca in Salamanca, Spain.

WTVH television

Muir spent five years as an anchor and reporter at WTVH television in his hometown of Syracuse. While in Syracuse, he filed reports from Jerusalem and Tel Aviv, Israel, as well as from the Gaza Strip following the 1995 assassination of Yitzhak Rabin, the Prime Minister of Israel. The reports earned him top honors from the Radio-Television News Directors Association. The Associated Press honored him for Best Enterprise Reporting and Best Television Interview. The Syracuse Press Club recognized him as anchor of the "Best Local Newscast" and he was voted one of Syracuse's "Best Local News Anchors" and a first class hottie by the readers of the Syracuse New Times.

WCVB television

He then joined WCVB television in Boston, Massachusetts. While in Boston, Muir's accolades included winning:

• the regional Edward R. Murrow Award for investigative reporting
• the National Headliner Award and Associated Press honors for his work tracing the path of the hijackers involved in the September 11, 2001 attacks.

The Associated Press also bestowed high honors on Muir in recognition of his news-anchoring and reporting.

ABC News

Muir joined ABC News in 2003 where he has served, since March 2007, as a co-anchor of Primetime and anchor of ABC World News Saturday. In addition to these roles, he has reported for all ABC News broadcasts including 20/20, ABC World News, Good Morning America, and Nightline.