Elaine Summers, 54, and Shaheen Kadri, 44, have been convicted of taking money for cleaning work that was never carried out for the RHS.
Summers, of Notting Hill, west London, was found guilty of stealing £360,859.51 between January 2004 and January 2014.
Kadri, of Camden, north London, was convicted on Friday of stealing £317,110 during the same time after a trial at Southwark crown court.
John Traversi, prosecuting, said: "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."
The three defendants will be sentenced on 9 September.
Judge Mr Recorder Gopal Hooper said: "The likely sentence of this kind of offence is a custodial sentence.
"There is a large amount of money involved over a long period of time.
"I do not want you to be under any illusions that by putting this case back for pre-sentence reports and granting you bail that I am giving you any indication of what the sentence is likely to be."
RHS director general Sue Biggs said: "The RHS uses donations and funds from its supporters to enrich lives and improve horticulture and the environment across the UK. For someone to commit fraud and take money from a charity is deplorable.
"This fraud was detected as over the last four years we have been conducting in-depth audits across all our work and implementing more robust measures and controls to protect the Society. Today we’re in a strong position to ensure unacceptable activity could not take place. The losses to the revenue of the RHS have been largely covered by insurance.
"We routinely monitor our work and look at all our procedures and controls to safeguard the RHS, its employees and volunteers and the interests of people who support us. We conduct audits across all our work and have further enhanced our financial controls to give even greater protection to the Society’s funds."
The RHS would not comment on who was auditing the activities of the department when the crimes took place.