As far as I know he was executed for murder in Utah in 1915. This is what Joe Klein has to say about him in his biography of Woody Guthrie:

[1980:] Joe Hill [Joseph Hillstrom, né Joel Haaglund] was probably more rogue than radical, and the songs he wrote were often little more than doggerel ... but he came to symbolize the spirit of the Wobblies in the public mind, mostly because of the phenomenal success he achieved in orchestrating his own martyrdom. After arriving from Sweden in 1902, he wandered through the West for the next thirteen years. Not much was known about him except that he often hung around the San Pedro I.W.W. hall. Some of the old-timers suspected he made his living as a robber, and only used the Wobblies as a social club. Then, in 1914, he was arrested in Utah for the murder of a market owner, a murder he probably didn't commit. But he refused to say where he'd been at the time of the shooting (to protect the honor of a lady, it was said), and was convicted. In the years that followed, Joe Hill became a cause célèbre among radicals and liberals all over the world. He did some of his most inspired work while awaiting his death, including the famous line he sent to Big Bill Haywood, the Wobbly leader, in a telegram just before his execution: "Don't waste any time in mourning. Organize." (Klein, Woody Guthrie 82ff)
And a less sceptical bit by Ailie Munro:
[1984:] At first a rather marginal figure in the I.W.W. struggles, [Joe Hill] was known chiefly for his songs which came to be sung across the world and were linked with working-class agitation as far afield as Australia. In 1914 he was arrested in Salt Lake City, Utah, on a murder charge, convicted on highly circumstantial evidence, and executed after 22 months in prison - despite an international defence movement, and petitions which included two pleas from President Wilson and one from the Swedish minister for further consideration of his case. The grim story of his trial by a hostile court, and the outcome, can be read in Barry Stavis's 'The Man Who Never Died'; written after five years of research into the facts, it fully endorses Joe's claim that he was framed as an anti-union, anti-I.W.W. move. This claim is also supported by the Labour historian Foner.
Joe's last message to his friends was "Don't mourn for me - organise". And his last will, written in the death-cell the night before he was shot, has a timeless nobility:
Joe Hill's (Last) Will
Set to music by Ethel Raim in 1961, as Joe Hill's Will.
Joe's body was reduced to ashes, which were placed in many small envelopes: "These were sent to I.W.W. ... sympathisers in all forty-eight states of the U.S. except ... Utah", and to many other countries throughout the world, to be scattered over the earth on May 1, 1916. But the Harvard-educated revolutionary John Reed wrote, "I have met men carrying next their hearts, in the pockets of their working clothes, little bottles with some of Joe Hill's ashes in them." His funeral in Chicago was attended by an estimated 30,000 sympathisers, who marched through the streets to the cemetery.
Some twenty years later, Alfred Hayes and Earl Robinson wrote this song. [...] Robinson's fine tune is in the hymn-like style [...] which was popular among Labour songs up till the forties and the fifties.
In the sixties, the English composer Alan Bush based his fourth opera on the life and death of Joe Hill as told by Barry Stavis. 'Joe Hill: the Man Who Never Died' was first performed at the German State Opera House, East Berlin, in September 1970 and ran for the whole winter season. (Munro, Revival 27f)