Haggard died Wednesday, his birthday, of complications from pneumonia at his home in Northern California, his agent Lance Roberts told CNN.
Haggard recorded more than three dozen No. 1 country hits in a musical career that spanned six decades, from the 1960s into the 2010s. He overcame an early life of petty crime and a prison term in San Quentin to develop a rugged, outlaw image that helped sell millions of records.
Tributes immediately began pouring in from the country music world and beyond.
"Country music has suffered one of the greatest losses it will ever experience," said country star Charlie Daniels on Twitter. "Rest in peace Merle Haggard."
"Love and prayers for the Haggard family. Merle was a pioneer ... a true entertainer ... a legend," added Carrie Underwood. "There will never be another like him."
"There are no words to describe the loss & sorrow felt within all of music with the passing of Merle Haggard. Thank God for his life & songs," said singer Brad Paisley.
His longtime friend, Willie Nelson -- with whom Haggard sang "Pancho and Lefty" -- tweeted a simple black-and-white photo with the lines, "He was my brother, my friend. I will miss him."
'The best country singer'
Haggard didn't just sing about the life described in country songs. He lived it.
His father died when Haggard was a child, and he ran away from home and later served time in prison. He drank -- one of his best-known songs is called "I Think I'll Just Stay Here and Drink" -- and partied. He was married five times.
Haggard's song titles were plainspoken and evocative. "I'm a Lonesome Fugitive." "Sing Me Back Home." "Branded Man." "The Bottle Let Me Down." "If We Make It Through December." He may not have written all of his hits, but he sang them with a pure feeling that left no doubt of the pain -- and the joy -- inside.
Despite his advancing age, Haggard maintained a robust touring schedule in recent years. He had canceled a handful of April concerts because of his declining health, and he predicted to his family that his end was near.
"A week ago dad told us he was gonna pass on his birthday, and he wasn't wrong. A hour ago he took his last breath surrounded by family and friends," son Ben Haggard posted on Facebook
"He loved everything about life and he loved that everyone of you gave him a chance with his music," the younger Haggard wrote. "He wasn't just a country singer.. He was the best country singer that ever lived."
A troubled youth
Merle Ronald Haggard was born in 1937 near Bakersfield, California. According to the bio on his website, his father worked as a carpenter for the Santa Fe Railroad and the family lived in an old boxcar that they converted into a home.
Merle's father died when he was 9, which devastated and unmoored the boy. He turned to petty crime and spent his teenage years in and out of reform schools.
But the young Haggard also learned early on that he had a talent for music and found his idol in country singer Lefty Frizzell, whose honky-tonk style he imitated.
After being convicted of attempted robbery as a teenager, Haggard spent several years in California's San Quentin State Prison, where he heard Johnny Cash play and was talked out of trying to escape by his fellow inmates, who thought he had a future on the outside as a successful musician.
"Going to prison has one of a few effects," Haggard told Salon in 1999
. "It can make you worse, or it can make you understand and appreciate freedom. I learned to appreciate freedom when I didn't have any."
After being released in 1960, Haggard made a modest name for himself by playing small clubs and backing more established artists. He scored his first hit in 1966 with "The Fugitive," which, although he didn't write the song, reflected his recent past as a convict.
Along with Buck Owens and Wynn Stewart, Haggard was one of the notables who created the "Bakersfield sound," which used electric instruments and a strong beat in country settings.
Although he was initially leery of publicizing his criminal past, Haggard eventually heeded friend Johnny Cash's advice to write songs about the darker chapters of his life. One hit, "Mama Tried," was an apology of sorts to his hard-working mother for his rebellious youth.
The story behind 'Okie'
Haggard's description of 1969's "Okie from Muskogee," perhaps his most famous song, changed over the years.
The tune, told from the point of view of a straight-arrow Oklahoman who praises it as a place "where even squares can have a ball" and disparages the "hippies out in San Francisco," was immediately adopted by what became known as the "silent majority," conservative middle Americans who wondered what was going on in an America out of control.
Haggard said that the song started out as a joke as his band was making its way across Oklahoma, and told Rolling Stone
that "the reason I wrote it is was because I was dumb as a rock."
However, he told the Boot in 2010
that the song was also meant to be patriotic.
"We were in a wonderful time in America and music was in a wonderful place," he said. "America was at its peak and what the hell did these kids have to complain about? These soldiers were giving up their freedom and lives to make sure others could stay free. I wrote the song to support those soldiers."
By the '70s and '80s, Haggard was churning out hits, appearing in TV specials and dueting with everyone from George Jones and Willie Nelson to Clint Eastwood (on a song from the "Bronco Billy" soundtrack).
Like many country stars of his era, Haggard struggled for hits in the late '80s and into the '90s. In 2000, he even signed with Anti Records, a label that leaned toward youthful punk, R&B and reggae. Of course, the anti-establishment Haggard fit right in, though he only stayed on Anti for two albums.
He finally returned to the top of the charts with Nelson. The pair's "Django & Jimmie" hit No. 1 on the country album charts in 2015. A single, "It's All Going to Pot," hit the country top 50.
Despite his misspent youth, four failed marriages and battles with drugs and alcohol, Haggard had a rosy view of his own life in a 2012 interview with CNN.
"I am smart enough to know that I have been gifted and have had a better than average shot at everything," he said. "I have been blessed many times."