Fighting in front of a hometown crowd, Jonas was forced to dig deep against Mayer, who was making her debut in the division.
In the early rounds, Mayer – a former super-featherweight champion – was chasing Jonas, whose wise shot selection and naturally greater power was proving the difference.
However, the American challenger began to find her rhythm as the bout progressed, varying her offence and backing up Jonas, 39.
Mayer, 33, seemed to be pulling ahead, leaving Jonas arguably needing to win the final few rounds to emerge from the fight with her championship status intact.
What followed was an enthralling series of rounds, which could have gone either way, as each boxer brutalised the other to the body.
Ultimately, Jonas emerged a split-decision winner, as the scorecards read 96-94, 93-97, 96-95 in favour of the two-weight champion.
“I know this girl here, Mikaela Mayer, is going to be absolutely devastated – because I’ve been there,” said Jonas. “Please, please use that as motivation to go on and get the title that you deserve and to become a two-division champion.”
Mayer added: “I thought I did enough to win. I would’ve given her the first round or two, but after that I think I outpunched her and landed the cleaner shots. Thank you to everyone who showed out tonight. I didn’t have a rematch clause on my side, but I hope we can [do this again].”
Meanwhile, Jonas – an Olympic veteran like her opponent – dismissed the suggestion that she might retire, but she did admit: “This is probably my last year in boxing, and we want it to be the best chapter in the book.”