Around 750 volunteers have clocked up over 6,200 hours in the past 12 months - the equivalent to an extra 3.5 full time members of staff.
Volunteers carry out nature conservation, ecological monitoring, path repairs and litter picking at both Burnham Beeches and Stoke Common.
And for the fourth year in a row they undertook a whole site reptile survey at Stoke Common over a 10 week period this Spring.
Mountevans said: "Burnham Beeches and Stoke Common are sites of great environmental and societal value. The sterling efforts of our staff and volunteers are making a real difference to the health and wellbeing of the local community, and are an important investment in our precious natural landscape. We owe a huge debt of gratitude to those who work so hard to protect these important green spaces."
The Lord Mayor heads the City of London Corporation, which protects and conserves 18 green spaces in London and south east England, including Hampstead Heath, Epping Forest, and over 200 smaller sites in the Square Mile.
It invests £29m a year in these green spaces which attract over 23 million visits every year, and are protected from being built on by special legislation.