More than 60 firefighters were called to 1100 S. Shamrock Ave., just south of Huntington Drive, at 5:40 p.m.
Monrovia Fire Chief Chris Marks said the roof had collapsed and the walls buckled Marks said that within 10 minutes after the fire began.
"When we ran out, the flames ran after us," said Chris Marks, who works at Jan-Kens, one of the two business that went up in flames."That's how bit it got that quick."
The fire was knocked down just after 7:30 p.m.
No injuries were reported.
The cause of the fire and a damage estimate were not immediately available.
Both businesses have been in business more than 40 years.
City News Service contributed to this report.