Rosemary Carroll, who made a post comparing an Asian with a dog last June, rejoined “as the votes were being counted” on Friday, giving the Tories narrow control of Pendle council in Lancashire.
Conservative group leader Paul White said: “The post was shared in error but Rosemary fully accepted the potential upset caused and sincerely apologised.
“Having served her suspension period she rejoined the party and completed additional diversity training.”
Ms Carroll, a former mayor of Pendle who represents the Earby ward, previously said she meant to delete the post but accidentally published it in error.
Leader of the Labour group, Mohammed Iqbal, 47, said the situation was an “appalling state of affairs”.
He said: “Here’s a councillor who brought shame on the borough on an international level and was welcomed back into the fold with open arms simply to grab control to the council.
“She turned up with a Conservative rosette literally as the votes were being counted.”
Ms Carroll was suspended from the party for three months over the post.
The political balance of the council is now Conservatives, 25 seats; Labour 15 seats, Liberal Democrats nine seats.
Mr Iqbal added: “If it was a fair and square result I could take it on the chin but this is shocking.”
The council said it was told Ms Carroll had rejoined the party at lunchtime on Friday.