There are a giant range of fields that use elements of math and biology. This could be quantitative biology, mathematical biology, systems biology, biophysics, bioinformatics, bioengineering, etc. All of these have slightly different cultures and questions they're interested in, and you might enjoy any of them. There are programs with undergraduate degrees in many of these hybrid fields, which could suit you.
Fomite's answer shows that you can start in either direction and end up in a combination. But my recommendation would be, if you are interested in working in academia in a position that combines mathematics and biology, it might be easier to ensure your undergraduate major has at least some mathematical elements, i.e. choose either mathematics or a hybrid major with math/physics/engineering.
One reason is (possibly unfair) stereotypes about biologists and mathematicians. The stereotype about mathematicians + physicists is that they don't care about the biological details. But the stereotype about biologists is that they don't understand the mathematics. This makes it harder for people with a biology background to be hired in a mathematically-driven group. (Harder - but not impossible!)
The less cynical reason to start with at least some mathematics is about depth vs breadth. Biology as a subject has colossal breadth, which can be wonderful! But it means that one great topic (say ecology) is not necessarily a prerequisite for another (cell biology). But on the mathematics side, if you want to use partial differential equations to model a biological system, you probably need calculus, linear algebra, ordinary differential equations, etc. first! I think, therefore, the track from high-school-level skills to research-level skills in mathematical biology is longer in mathematics than it is in biology.