Seeger's grandson, Kitama Cahill-Jackson, says Seeger died Monday night after being hospitalized for six days.
Seeger gained fame as a member of The Weavers, the quartet formed in 1948 and had hits such as "Goodnight Irene."
He continued performing and recording for six decades afterward and was still an activist as recently as October 2011 when he marched in New York City as part of the Occupy Wall Street protests.
He was onstage in January 2009 for a gala Washington concert two days before Barack Obama was inaugurated.
But in the 1950s, his leftist politics got him blacklisted and he was kept off commercial television for more than a decade.