Baumgardner added WBO and IBF titles to her WBC super-featherweight belt with the win. The scores were tallied with two of 96-95 in Baumgardner's favour, with the third judge being wildly different at 97-93 in favour of Mayer.
The American caught the eye early on with a devastating counter-attacking style, launching punches off her back foot whenever she saw an opportunity.
But Mayer fought well, especially in the middle rounds, and it was easy to see why the judges weren't able to come to the same conclusion.
Despite the build-up to the fight being quite heated, the opening round was very tentative by both fighters, with both perhaps looking to avoid an early mistake and embarrasing loss.
The first big punch was landed in the second round when Baumgardner launched an assault and breeched Mayer's defences with a nice left hook and a strong follow-up upper cut.
Mayer's fightback into the bought began in the third with he contant testing of Baumgardner's defences starting to find some weaknesses.
She pretty much controlled the fight from that point until the end of the fifth when she was able to cage Baumgardner into a corner.
Hovwever, Baumgarder had enough by the start of the sixth and came back with a sharp jab to expose a cut above Mayer's right eye that forced her into a much more defensive posture.
Baumgardner recognised her advantage and took control of the fight till the end.
It was a close fight but the fact that Baumgardner was so dominant at the start and at the end is probably what swayed two of the three judges to rule in her favour.