[6] Mayer beat Ewa Brodnicka by unanimous decision to win the WBO female super-featherweight World title at the MGM Grand in Las Vegas on 31 October 2020.

She lost via split decision with judges John Latham and Steve Gray making Baumgardner the winner, both scoring the fight 96-95, while Terry O’Connor gave it to Mayer 97-93.

Sweden's Lucy Wildheart stepped in as a last-minute replacement with Mayer winning by unanimous decision to became the interim WBC female lightweight World champion.

[10] Another change in weight division saw Mayer challenge IBF female welterweight World champion Natasha Jonas in Liverpool, England, on 20 January 2024.

After a closely fought contest, it was the English boxer who came out on top winning by split decision with judge Diana Drews Milano scoring the fight for Mayer 97-93 but Frank Lombardi (96-94) and Michael Alexander (96-95) giving it to Jonas.