Andrea Cochrane made it look like cash was being paid to the firm's accountants when it was going straight into her own bank account.
Mum of two Cochrane, 51, of Millbank Avenue, Bellshill, appeared at Hamilton Sheriff Court on Friday.
She admitted embezzling the money from A. Steel and Son Contracts Limited, of Jackton Farm, Eaglesham, between January 2018 and June the following year.
Mairi-Clare McMillan, prosecuting, said Cochrane had been employed by the company for 31 years when the crime came to light.
Her duties including dealing with payments for customers and staff.
A member of the family who own the business had noted anomalies in the financial records.
These included a £2,500 payment to its accountants.
Cochrane was asked about this and claimed it was an over payment.
Her explanation was accepted as "plausible", the fiscal stated, but further investigations were made.
The company records showed that on another occasion £28,000 had apparently been paid to the accountants when the payment should have been only £10,000.
Ms McMillan said: "The company's bank provided what it said were the account details for the accountants. However, the account number and sort code were for the accused's personal bank account. It was later confirmed that a total of £90,000 had been paid into her account."
The police were alerted and almost immediately Cochrane instructed her lawyer to write to the company, accepting responsibility for the missing money and apologising for what she had done. She offered to pay £200 per month "until all the debt is paid off". Defence agent Alan Murray said his client felt "great distress" for the family-owned company.
Bankruptcy proceedings against Cochrane are underway and her home is being sold. Her husband does not work, but she now has a job in a bakery.
Sheriff John Speir deferred sentence on Cochrane, who has no previous convictions, for next month for a social work report.