Roberts was born in Toronto, Ontario and grew up in Mississauga, Ontario. He attended both Lorne Park Secondary School and Erindale Secondary School and later the University of Toronto at Mississauga, and worked for many years in Canadian media before joining CBS News in 1992.

Roberts first started working in radio at the local college station CFRE-FM, in Mississauga, Ontario. He briefly worked on-air on radio station CJBK in London, Ontario in 1977, moving back to Toronto to top-40 powerhouse CHUM that year, where he was heard as weekday evening DJ. In 1979, he branched out from his music personality work to function as an entertainment reporter and co-host with Jeanne Beker of the music newsmagazine The NewMusic until 1985. Roberts also worked briefly with CFOS in Owen Sound, Ontario in the 1980s. He was a video jockey as fill-in host of CityTv's Toronto Rocks, and later on CityTV-owned MuchMusic, a music video channel. At MuchMusic he and Christopher Ward appeared as the first on-air personalities when the network launched in 1984. At MuchMusic he also hosted a one hour Heavy Metal video show called "The Power Hour." He also served as an entertainment reporter and anchor on CITY-TV's CityPulse, and became anchor of CityPulse Tonight in 1987 when Anne Mroczkowski moved from anchoring that newscast to join Gord Martineau on the 6 pm edition.

Starting in 1989, Roberts began his 17-year association with CBS television and radio. Between 1989 and 1990 Roberts was an anchor/reporter for WCIX-TV (now WFOR-TV), the CBS-owned and operated television station in Miami, Florida. Roberts returned to Canada in September 1990 to co-host the national CTV morning show Canada AM, initially with Deborah McGregor, and later with Pamela Wallin. He also anchored the 5 and 11pm newscasts at CBS's flagship station in New York, WCBS-TV.

In March 2003, Roberts was embedded with the 2nd Light Armored Reconnaissance Battalion of the U.S. Marines during the initial invasion of Iraq. In October 2006, he returned to Iraq to anchor several editions of This Week At War and report for the Special Investigations Unit documentary "Death Squads".

On July 19, 2005 he introduced CBS's coverage on the announcement of the nomination of John G. Roberts, Jr. for the Supreme Court of the United States. He once joked in a newspaper column that he asked to be referred to as "Your Honor" because he and Justice Roberts bear the same name.

Soon after, Roberts left CBS and joined CNN. Together with CNN colleague Anderson Cooper, he won critical praise for his work covering the Hurricane Katrina disaster which devastated New Orleans and the American Gulf Coast. He and Anderson have formed a close relationship.

In July 2006 and August 2006, Roberts reported from the front lines in the Israel/Hezbollah war and at one point, embedded with an Israeli infantry unit, marching 10 miles into Lebanon. CNN's coverage of the war was recognized with an Edward R. Murrow Award.

On March 12, 2009, Roberts was inducted into the Canadian Broadcast Hall of Fame.

Divorced from his wife in 2008, Roberts has been dating fellow CNN anchor Kyra Phillips as of March, 2009.

On April 25th, 2010 John Roberts announced his engagement to CNN anchor Kyra Phillips