Allen, a convicted murderer had been on death row for 23 years. Just after midnight, the condemned man was led to the so-called death chamber. Following a lethal injection he was pronounced dead at 12:38 a.m.
The 50 witnesses included Allen's friends and relatives and those associated with the three victims he ordered killed in 1980.
Prison spokesman Eric Messick says the inmate spent most of his last day with visitors.
"I don't have a precise number, but it was quite a few. He was allowed five at a time and the rotations continued throughout the day."
Monday afternoon the U-S Supreme Court denied a request for a stay-of-execution. Last week Governor Schwarzenegger refused to grant clemency.
At 76, Allen is the oldest inmate ever executed in California.