Ryan began slowly in opposition to Mayer, who used her trademark velocity and skill off the jab to construct an early lead.
The defending champion got here again within the sixth spherical, touchdown an enormous left hand quickly after.
Ryan had one other massive spherical within the eighth, however Mayer was as much as the problem of her largest photographs and stood agency.
Derby’s Ryan, who now has two defeats and 7 wins, stated the paint incident “threw me a bit off my sport plan”.
“You noticed at the beginning of the battle, I used to be dealing with her fairly effectively on the jab,” she added.
“I used to be boxing her after which I began to choose it up. However then, I used to be clearly pushing it too arduous.”
Mayer turned a two-weight world champion with the victory, having beforehand been world champion at super-featherweight.
The American’s transfer to welterweight has been questioned due to the 17lb weight leap, however Mayer, who suffered a controversial defeat by Natasha Jonas in January and has now crushed the most effective welterweights within the division.
“[Ryan] simply couldn’t deal with my timing, so, I felt like I received the battle, and I’m simply glad I received the precise choice this time,” she stated, referencing the Jonas bout.
“I wish to end what I began and go undisputed. I got here very shut at [super-featherweight].
“However my physique outgrew that weight. I’m snug now. I really feel like I’m stronger than ever. I’m nonetheless in my prime, as you may see.
“So I’m coming for the opposite champions.”
