Susan Rennie, 51, engineered 105 fraudulent payments into her personal account by doctoring company invoices, a court heard.
She used the stolen cash to pay off her son's £200,000 drug debt and spent the rest on holidays and cars.
Rennie pleaded guilty to fraud by abuse of position and was jailed for four-and-a-half years at Bolton Crown Court.
The mother-of-three, who had previous convictions for benefit fraud and theft from a previous employer, worked as a purchase ledger for IT provider Ascribe Ltd.
She moved to Lytham, Lancashire, while on a suspended sentence, and began working for the firm in 2007.
'Desperate addiction'
The fraud was uncovered after Rennie's resignation, when the firm realised profits were down and launched an investigation.
Passing sentence, Judge Graeme Smith said Rennie "appeared to have an addiction to taking money".
"Once discovered, the fraud was easily traced back to you - there was no attempt to cover your tracks," he said.
"You chose not to stop your offending but continued it up to the point of resignation, increasing the amount you took."
As a result of the fraud, the company was sold by Ascribe Ltd at a reduced rate, the court was told.
Defending, Rachel Woods said: "The initial motive for this fraud was due to her 18-year-old son, who had a desperate addiction to cocaine."
A fraudster with an "addiction to money" who stole nearly £600,000 from her employer has been jailed.