The aquaculture and genetics firm said Plampin would continue in the role until a successor was found to ensure an orderly transition process.
Benchmark added that an external search for a new CFO had commenced and a further announcement would be made “in due course”.
“I am proud of the significant milestones achieved during my time at Benchmark. We now have a focused pipeline of products and the infrastructure in place to support the Group's sustainable growth. I have enjoyed being part of a strong team at Benchmark and I look forward to working with my successor to ensure a smooth handover”, Plampin said.
Benchmark's Chairman, Peter George, added: "On behalf of the Board, I would like to take this opportunity to thank Mark for his considerable contribution during his time with the Group and wish him every success in the future.
"We have begun the search for his successor and look forward to updating on progress and ensuring a smooth handover of duties in due course."
In early trading on Tuesday, Benchmark shares were 1.8% lower at 49.6p.