Terry has served as editor of the Times for 22 years, and marked four decades with the paper in October 2015.
His redundancy follows a management shake-up at publisher Archant’s Hertfordshire titles.
He posted: “Certain @whtimes will hugely miss outstanding local editor Terry Mitchinson. Good luck Terry, you’ve served Welwyn Hatfield [with] distinction.”
Terry is the third senior editorial figure to leave Archant in recent months.
Last week Eastern Daily Press editor Nigel Pickover announced he would be retiring at Christmas after 44 years in the industry while another experienced newsman, North London editor-in-chief Geoff Martin, has also recently stepped down.
During his time as editor, Terry helped launch two now-defunct newspapers, the Comet Weekender and the Leader series, and helped launch Archant’s Hertfordshire Life magazine.
The Times was one of the first newsrooms to trial Archant’s content management system, which has since been introduced across the group.
An Archant spokewoman said: “We can confirm that Terry’s role has been made redundant following a reorganisation of the editorial management of Archant’s Hertfordshire titles.”