The role of company secretary will be filled on an interim basis by David Farrington, who has worked for a number of charities in the past, including Royal National Institute for Deaf People and Macmillan Cancer Support.
RHS head of press Lynn Beddoe said: "No-one was refused voluntary redundancy because it improves opportunities for redeployment. This has been planned for six months."
She added: "This is an influential role and [RHS president] Giles Coode-Adams wanted to send round a note saying thank you and farewell."
Smith is taking a break before taking a new job.
Meanwhile, Wisley fruit and trials supervisor Andrew McSeveney has replaced Fiona Wilson as RHS principal trials officer. Beddoe said: "This would have been advertised if we had not had a successful internal candidate."
The RHS has lost 131 staff in recent redundancy procedure and is now advertising for 30 new staff.
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