John Coyle tried to rob a Scotmid store in Glasgow's Easterhouse, but left empty-handed after the worker shouted to her mother who also worked there.
The High Court in Glasgow heard that Coyle's "very distinctive" hair could be seen on the footage.
The 43-year-old, from Cranhill, will be sentenced in the new year.
It emerged Coyle had served a number of lengthy jail terms for similar crimes.
He had only recently been released after being jailed for a robbery in 2009 before the latest attack in October.
A serial robber who threatened an 18-year-old shop worker with a knife was caught after his streak of grey hair was recognised in CCTV footage.