Ms Armstrong has been in the role since August 2020.
C4EM board chair Dean Oberin paid tribute to Ms Armstrong’s contribution.
“We wish Deanne well in her future pursuits and thank Deanne for her efforts as the CEO,” Mr Oberin said.
“She was influential in developing an efficient operational organisation with a clear vision and high-level engagement with local government, agencies, members, and wider communities.”
Mr Oberin said Ms Armstrong had made a significant impact on the organisation.
“During this challenging time, we have navigated COVID and floods in our community where Deanne has been instrumental in helping the organisation define and rebrand, further strengthening community connections, and refocus on its strategic direction and priority initiatives for our future,” he said.
“We now have a strong foundation in place, a refreshed new look, especially around projects, and a strong growing membership base.
“This also provides a sound future-focused basis to facilitate effective advocacy for our community's future needs.
“With these ‘building blocks in place, C4EM is in a great position for someone to build on Deanne’s legacy.”
Ms Armstrong will continue in the role until February 3 and support the search for a new chief executive officer.