Montimore will replace Gareth Rees, who resigned earlier this year after five years in charge of the FRC's disciplinary process.
The new interim executive counsel will start in the role on 1 November, the FRC said.
After being called to the Bar in 1994, she practised at Red Lion Chambers for 10 years before becoming a prosecutor at both the Revenue and Customs Prosecutions Office (RCPO) and the Department for Business. She has been working at the regulator since 2013.
A FRC spokesperson said that she had been appointed on an interim basis to continue the work and leadership of the enforcement division, while the process to recruit a permanent counsel is underway.
It is understood Rees resigned to go back into private practice. According to a report from Sky News, he is set to join US law firm King & Spalding as a partner.
Back in July, the accountancy regulator appointed Mary Cleary to its Audit and Assurance Council and John Hitchins to its Conduct Committee.