Andrew Stubbs, 41, forged more than 2,000 cheques over eight years while working for the London cost consultant to pay his gambling debts.
Southwark Crown Court heard Stubbs raised cheques for himself, exploiting the company system where there were no mechanisms for checking sums under £1,000.
His actions almost bankrupted Leslie Clarke, which had to lay off staff, and refinance the firm, reported the Daily Mail.
The fraud came to light when HM Customs and Revenue issued a demand for unpaid tax.
Judge Jeffrey Pegden QC at Southwark Crown Court, said that it was difficult to overstate the impact that Stubbs’ criminal offending had on the partners and employees of the firm.
Jailing Stubbs he said that the £1.8m loss nearly had a disastrous effect upon the firm.