The man, whose name was not reported, calmly entered the store on Azusa Avenue before 1 p.m. Wednesday and grabbed a handsaw, the West Covina Police Department said.
"He just began sawing away," Cpl. Rudy Lopez told the Los Angeles Times. "He was pretty much intent on doing what he did."
An off-duty Pasadena paramedic and store employees used rags and twine to fashion tourniquets and stem the heavy bleeding, Lopez said.
The man was passing out as police arrived, he added.
Paramedics took the man to the nearby Queen of the Valley Hospital where he underwent surgery, the Times reported. His condition was not reported.
Lopez said the Home Depot was shut down for the rest of the day.