He is credited with helping to define the “Bakersfield sound”, country music with a distinctive twang and lack of sophisticated production. Haggard wrote songs, sang and played fiddle and guitar, usually singing about American pride and outlaws.
He was a member of the Country Music Hall of Fame with dozens of albums and top hits.
The singer, who battled lung cancer in 2008, had recently cancelled April tour dates due to illness and said he hoped to be back on the road in May.
Rolling Stone magazine has described him as “the backbone of one of the greatest repertoires an all of American music, plain-spoken songs populated by the kinds of working people Haggard grew up with: farmers, hobos, convicts, widows, musicians and drunks”.
His top hits include Mama Tried, The Fugitive, If We Make It Through December and Okie from Muskogee.
He was born in California in 1937, living in a converted boxcar with his family and quitting school at a young age to hop freight trains across the south western part of the US.
He spent three years in prison for breaking into a cafe while drunk. Then-governor of California, Ronald Reagan, expunged his criminal record in 1972.
Haggard’s former publicist told the BBC that the singer was as influential as the Beatles in country music.
He was the bottom line in country music and the gold standard to which every country artist measured himself, he said. “He was a nice guy with a great sense of humour. He was a very literate man.”
Musicians gave their tributes to Haggar shortly following the news of his death.
Country musician Luke Bryan tweeted that “a true hero was lost”.