Chief exec Sue Noyes was due to stay until June 2016, however her personal circumstances have changed.
Sue’s last working day at EMAS will now be this Thursday (March 17).
Pauline Tagg, chairman of EMAS said: “I had already started discussions with NHS Improvement about our future leadership arrangements and we are exploring options with them.
“In the meantime, our director of operations, Richard Henderson will become our acting chief executive when Sue leaves EMAS.
“When Sue joined EMAS two and a half years ago it was in a very different place. There are still many challenges to face but a lot has been achieved since then.
“Sue has led the development of a number of improvements including re-establishing the ambulance technician role, improving staff support by creating our Peer to Peer and Pastoral Care Workers programmes, and investing in 29 new ambulance vehicles to support our frontline crews.”
Ms Tagg added: “Last year we started the biggest recruitment drive ever seen at EMAS, adding 300 new frontline staff to our service. “More lives are being saved through our cardiac strategy, with the increase in return of spontaneous circulation for patients in cardiac arrest, and the introduction of Paramedic Pathfinder has helped us improve the urgent care we provide.
“Sue has plenty to be proud of from her time at EMAS, and we will be sorry to say goodbye.”
Ms Noyes said: “Leaving has been a very difficult decision, and I am incredibly proud to have been part of the EMAS team.
“Colleagues across the service are tremendously dedicated, and work incredibly hard to help our patients. Despite the challenges, I know they will continue to work with others to bring about further improvements for local people.”