Medhurst, who left the RHS in 2013, conspired to steal nearly £700,000 from the charity over a 10-year period.
He had appeared at Westminster Magistrates Court in January 2015 alongside Shaheen Kadri and Elaine Summers, neither of whom worked for the charity, before the case was progressed to the Crown Court, where the 55-year-old, of no fixed address, pleaded guilty to one count of fraud by abuse of position and two of conspiracy to steal.
Kadri, 44, and Summers, 54, also appeared at Southwark and pleaded not guilty to a single charge each of conspiracy to steal.
Medhurst will be sentenced after the conclusion of their trial, which began on 11 July and is due to last between five and seven days.
Between January 2004 and January 2014 Summers, of Pembridge Gardens, Notting Hill, is charged with stealing £360,859.51, while Kadri, of of Oval Road, Camden, is charged with theft of £317,110.
Prosecutor John Traversi told the jury at Southwark Crown Court on 11 July: "Stuart Medhurst, in collusion with Elaine Summers on the one hand and Shanheen Kadri on the other, submitted false invoices prepared by Ms Summers and Mr Kadri for cleaning work that was never done."
He said invoices were made for payments for work including cleaning and maintenance but suspicions began in October 2013, when "questions arose over invoices that had been paid, invoices which emanated from a company named EV Summers for cleaning services".
He added: "No legitimate work was carried out on behalf of or by Elaine Summers and the payments made into her account were a fraud, engineered by the issuing of false purchase orders by Stuart Medhurst."
He said during the 10-year-period another company had actually been providing cleaning services to the RHS.
Work claimed to have been carried out by Kadri included maintenance work to the value of almost £6,000 at Lawrence Hall in London.
Of the defendant, Traversi said: "No such work was carried out by or on behalf of Shaheen Kadri and a loss to that degree was suffered by the RHS."
Both Summers and Kadri deny the single count of theft against them.
The trial continues.