Barby Dashwood-Morris admitted breaching planning law before Brighton magistrates earlier this month.
Wealden District Council leader Bob Standley said she was disqualified as a member after not attending a meeting for six months.
Ms Dashwood-Morris has not yet commented to the BBC.
Mr Standley said: "The rules are quite clear - if a member does not turn up to a meeting for six months then they're automatically disqualified."
He added: "When Barby Dashwood-Morris pleaded guilty to the charges, I would have preferred her to resign, but she didn't."
The council boss said Ms Dashwood-Morris was warned about her attendance, but said he understood she decided not to attend during the trial.
"While that was going on she decided not to engage in activities and that was the consequence. What was in her mind I can't speak for," he said.
During the court case, magistrates heard changes made to her 600-year-old Hellingly home, which included knocking down a wall and installing windows, were discovered when she tried to sell it.
At the time, she was chairman of Wealden's planning committee.
In a statement, Ms Dashwood-Morris told the court she and her former partner, Alan Proudfoot, who also admitted breaching planning law and was fined, "strongly believed" the work did not require authorisation.
A statement from Wealden District Council said: "A vacancy has arisen in the Chiddingly and East Hoathly Ward on Wealden District Council.
"This is due to the non-attendance of Barby Dashwood-Morris at any council or committee meetings over a successive six-month period which under Section 85 of the Local Government Act 1972 results in the immediate cessation of a councillor being a member of the authority."
The statement said Ms Dashwood-Morris was chairman of Planning Committee South from 2011 until 2013, when she was elected vice-chairman of the council.
A former council planning boss who was fined Â£75,000 for making illegal changes to her Grade II-listed Sussex home has been made to give up her post.