A male Lord Mayor of Cardiff named Roy Llewellyn was killed in the Auton invasion of Cardiff on 5 March 2005. His death had been announced to the public as being the result of the 'terrorist attack' that had occurred on that date, which was the government cover story for the Auton invasion.
However, Llewellyn had actually been murdered by a local councillor named Barry Jackson who desired the position of Mayor for himself, both for personal gain and acting on the orders of the Committee. Jackson had used the Auton invasion to cover up his crime and then proceeded to use an alien device that controlled an unidentified blue, monstrous creature to murder his rivals for the position of Mayor. Over the three weeks until 26 March 2005, the creature murdered five local councillors in a spate of beheadings on Jackson's orders. Despite the intervention of Yvonne Hartman on that night in attempting to stop the creature, it proceeded to murder Ross Bevan, Julie Jones and finally Helen Evans, leaving Jackson unopposed for the position of Mayor.
Hartman had worked out Jackson's plot and confronted him at his apartment, threatening to kill him for murdering innocent people and using alien technology to create scenarios that endangered the country. However, as she needed a spy in an official position to keep an eye on Torchwood Three and the Committee, she decided to let Jackson remain alive on the condition that he work for her. As a result, Jackson became Mayor of Cardiff. (AUDIO: One Rule)
Jackson was eventually succeeded in the middle of 2006 by Blon Fel-Fotch Passameer-Day Slitheen, posing as Margaret Blaine. The position was once again left vacant in September 2006 when Blon's plot to escape and destroy the Earth was foiled by the Ninth Doctor, who used the Heart of the TARDIS to regress her into an egg before returning her to Raxacoricofallapatorius. (TV: Boom Town)
By 2017, the city had been infiltrated by the Sorvix race during a period of inactivity for Torchwood. Their leader, Ro-Jedda, became Lord Mayor soon after and began to oversee Sorvix integration. (AUDIO: Orr, et al.)