The 25-year-old Englishman admitted recently that he had endured the toughest spell of his career after a torrid six months which saw him suffer a controversial defeat by Lamont Peterson and a bitter fall-out which saw their rematch scrapped after the American failed tested positive for synthetic testosterone.
Khan, who had perhaps naively talked about fighting Floyd Mayweather in the near future, had the WBA belt he lost that night returned to him but lost that to Garcia as the American caused a huge upset to retain his own WBC crown and become a unified champion at the Mandalay Bay Events Center.
Khan came into the fight with a record of 26-2 with 18 knockout wins while Garcia boasted 23 wins, 14 early, and no defeats.
Despite Khan coming off a defeat in his last bout, the Briton was a heavy betting favourite and the majority of the 7,000-strong crowd were in his corner.
Khan flew out of the blocks and barely a second had elapsed when he landed a beautiful left hook-right hand combination.
He easily won the first round with sharp, attacking jabs and hooks, while Garcia had to settle for two right crosses and a naughty low blow, for which he was warned.
Garcia landed a nice right in the second but Khan retaliated immediately. Khan also won the second but had to take sporadic body shots and a strong overhand right from the Philadelphian.
Garcia was cut over his right eye in the round, though it was unclear how the wound was inflicted.
Khan landed an excellent lead left in the third which Garcia shrugged off well, while the American was also having occasional successes himself. Another warning for low hitting was perhaps harsh.
The fight then exploded as a counter left from Garcia floored Khan hard. He got to his feet quickly but was clearly devoid of his senses.
Referee Kenny Bayless scandalously allowed it to go on - Khan had no idea where he was - and the Briton was saved by the bell as Garcia threw a massive right and a left on the bell had Khan struggling again.
After a minute to recover Khan looked in better shape for the fourth. What followed was one of the most brutal rounds in recent memory.
Garcia threw huge power shots in search of the stoppage and Khan was soon down again, this time from a right, which saw him trip on the rope.
Khan somehow survived and amazingly began firing shots back as the two men stood and traded crashing blows. Khan actually goaded Garcia in and landed a good shot himself, hitting Garcia with an uppercut and left.
Khan was then down a third time when he was caught by a glancing left and right to the temple. Again he got up quickly, but when Bayless stared into his eyes, the referee finally decided enough was enough.