Another 17 members of Khun Pian's governing Rak Patong (Love Patong) group have also stood aside, one of the three deputies said.
Deputy Mayor Chairat Sukban confirmed today that he and two other deputies, Somvhaai Boorarak and Prasarn Yordtor, would take over the mayor's duties.
Yesterday's verdict by the Administrative Court to nullify the results of last year's mayoral poll was expected.
Back on August 18, Mayor Pian told Phuketwan that if the court backed the Electoral Commission's finding, as seemed likely, he will stand again in any fresh poll for mayor.
Discrepancies found by a Phuket investigation and passed on to the national Electoral Commission include a large number of voters who used the same address.
A letter to Patong Council on August 8 confirmed that the national Electoral Commission had supported the findings of the Phuket Electoral Office.
''I'll accept what the court says,'' Mayor Keesin told Phuketwan back then. ''I have no problem with that.''
At the age of 70, he said, any election that followed the court's decision would probably be his last.
He has no intention, he said, of encouraging his children to go into politics when he gives politics away.
Mayor Pian, a force in local politics since 1994, won the poll in early September 2012 with 4352 votes. Rival candidate Kittisan Kuru and his Patong Kawna (Patong Development) group attracted 3154 votes.