What are you thinking of when you say straight mathematical disciplines? In any case I have one example:
Prior to my undergrad thesis work I thought geometric measure theory was totally unrelated to physics. It turns out that it’s actually very important in the existence theory of complete minimal hypersurfaces in hyperbolic space. Fast forward 30-40 years such work is being utilized in physics research pertaining to the black hole information paradox.
If you’re unfamiliar, it’s basically a discrepancy discovered by Hawking in which the quantum mechanical notion of unitary is violated inside a black hole.
Modern work by Engelhardt suggests that the information radiated in black-hole evaporation can be modeled/calculated with a more generalized notion of a quantum external surface. This is not a minimized of the area functional but rather the (area+bulk) functional where the bulk term accounts for the effects of “quantum” near your black hole. In fact the bulk term is the so called entanglement entropy of quantum fields you hear all the time In physics papers.
Now all of this is intimately related with AdS/CFT which was conjectured from looking at the the Poincare ball model of hyperbolic space H3! Thinking of the Riemann sphere as the conformal boundary, every orientation preserving isometry of H3 gives rise to a Mobius transform on the Riemann sphere and vice versa. The AdS/CFT conjecture /holography basically says that given some quantum gravity theory in your bulk space you can equivalent consider the conformal field theory its the boundary.