Paris has been with the association for seven years, working as vice-president, president and chief executive.
During her time at the head of the organisation, the HTA has restructured to improve member services, acquired and moved to a new headquarters and restructured its investment portfolio to reduce the risk and volatility of exposure to the stock market.
The HTA has collaborated with many other organisations and was a founding partner of the government Ornamental Horticulture Roundtable, raising the profile of ornamental horticulture in the political sphere.
She said: "I feel very privileged to have had the opportunity to serve the HTA. I am confident that as there is now an established senior team in place and a new HQ, it is time for me to move on to new challenges."
HTA president, Adam Wigglesworth thanked Paris for "her many years of enthusiasm and commitment to the HTA".
"We wish her much success in the future. The strong team Carol leaves at the HTA are ready and determined to continue driving membership satisfaction, benefits and promoting the garden industry," he said.