David Scott Mustaine was born on September 13, 1961, in La Mesa, California, to an abusive, alcoholic father and a victim mentality mother. Due to his father's eccentricities, the family moved around quite a bit during the 1960s and 1970s, though by the time Dave was 16, he was living on his own in Los Angeles. He was already versatile, though self-taught, with the guitar, having been influenced heavily by the works of Cat Stevens.

While he was living on his own, he dealt drugs for while to support himself. One of his clients was often short of cash and offered Dave record albums in trade for drugs, which is how he got his interest in heavy metal, the New Wave Of British Heavy Metal, in particular.

1981 found Dave looking to join or start a band, and he answered a newspaper ad placed by one Lars Ulrich, a drummer, and his singer/guitarist friend James Hetfield. Upon receiving directions to the house where Lars and James were living, Dave brought all his guitar equipment over, plugged in, and started warming up. Lars and James just watched for a while. Nervous, Dave asked "so are we jamming, or what?" Lars and James were so impressed just by Dave's warmup, they merely nodded and said "you've got the job," and thus (with the later addition of bassist Ron McGovney) Metallica, in its first incarnation, was born.

Metallica played the club circuit around Los Angeles for several months in 1981 and 1982, gaining a cult following, mostly based on the strength of their demo tape, No Life 'Til Leather.

On an off-night in 1982, the band attended a Trauma concert and were astonished by Cliff Burton's wild bass-playing skills. Eventually McGovney was kicked out of the band, or quit (depending on who you ask), and Cliff was asked to join, which he did upon the condition that the band relocate to his hometown, San Francisco.

The band moved to San Francisco and built up the same cult following they'd had in Los Angeles, only this time they attracted the attention of a record label, Megaforce Records. Megaforce was in New York City, so the band relocated again.

After a tumultuous cross-country drive in a U-Haul van, the band fired Dave upon arrival in New York City in the winter of 1983, due to his almost constant drunkenness, and the belligerence and recklessness that came with it. The band packed up Dave's gear, drove him to a Greyhound bus station, and put him on a bus back to Los Angeles.

Shortly after his return to Los Angeles, a crestfallen Dave met and befriended bassist David Ellefson, after hearing each other playing their respective instruments through the walls of the apartment complex they shared. The two considered forming a new band and began looking for members, finding them, and cycling through them due to personality conflicts, defections, and arrests. The original Megadeth (as the new band had been named) consisted of the two Daves, former (and future) Slayer guitarist Kerry King, and drummer Lee Rash. King and Rash exited abruptly, and were just as quickly replaced, just in time for Megadeth's debut album, Killing Is My Business... And Business Is Good, which was released on Capitol Records in 1985. Megadeth continued touring, recording, adding new members and firing old ones, and releasing consistently better and better albums.

During all this, Dave was a heroin addict, among other things. Apparently there was nothing he would not try, and few things he didn't. He had several near-death experiences due to heroin overdose, came clean a few times, but always ended up back on the scag eventually, even while covering the Democratic National Convention for MTV in 1992. He would often be too fucked up (either high or on remission) to shoot a video, or sing and play guitar at the same time, as evidenced in the appalling video for Megadeth's cover of Alice Cooper's "No More Mr. Nice Guy," which consisted largely of Dave strapped into a chair while making menacing faces. Finally, in 1995, he came clean for good, and the Megadeth metal machine was back in full force, releasing three albums, touring incessantly, and appearing on a handful of movie soundtracks between 1995 and 2001. During this time, Dave also tried his hand at acting, appearing semi-regularly on the Sci-Fi Channel's The Black Scorpion as the villain "Torchy Thompson."

Dave was also instrumental in bringing Megadeth to the world wide web in 1994, much earlier than most bands. He continues to maintain an active role in maintaining megadeth.com, which he has said has brought the band much closer to its fans than he ever would've imagined.

In early 2002, nerve damage (most likely caused by thousands of heroin injections along with Dave's extremely fast guitar-playing) was discovered in Dave's left arm, which was so severe that he could not continue playing guitar, effectively ending his life as a rock star, at least a guitar-playing rock star. The nerve damage turned out to be radial neuropathy, leaving the band with little choice but to disband, which they finally did on April 3, 2002.

Dave successfully recovered from surgery and is able to play guitar again. Megadeth reformed and released the half live, half studio album Rude Awakening in 2003.

Dave married Pam Casselberry in 1991, and they have three children, sons Justis David and Victor Gar (named for the band's mascot/album cover model Vic Rattlehead, and for Gar Samuelsson, a former Megadeth drummer who died in 1998), and daughter Electra Nicole.

From heavy metal icon to deluded conservative mouthpiece!

In 2004, Dave inexplicably became a born-again Christian. He's parlayed that into becoming a batshit insane birther-conservative douchebag, as illustrated in this video and this Rolling Stone article. I wonder what the Dave of the "Peace Sells" 1980s would think of his modern-day counterpart; probably not much, which is a shame. Dave used to be really intelligent—give 1992's Countdown to Extinction a listen. It's politically liberal, pro-environment and anti-corruption. The Dave of today is no different than any other frothing, conspiracy-obsessed, right-wing maniac and he may as well be recording the between-segment musical fanfare for Fox News. He's now every bit as obnoxious and ridiculous as Ted Nugent.

Sources:

Metallica Unbound: The Unofficial Biography by K.J. Doughton (1992)
http://www.megadeth.com/
http://www.allmusic.com/
http://us.imdb.com/name/nm0615962/