Mathematics is very much concerned with relationships and their magnitude in space and time. These relationships can be between anything that is related, e.g. arranging a collection of items from their smallest to their largest, sorting a collection of data according to a defined character of the data (its colour, energy, potential, size etc) and so forth. Philip Maini is a Northern Irish Professor for Mathematical Biology. He is the Head of the Centre of Mathematical Biology. He is an example of someone who is a mathematician and is bridging two disciplines.
Mathematicians also have to develop their subject and study relationships for their own sake (Pure Maths as distinct from Applied Maths). Many very practical problems have been solved thanks to solutions developed by pure mathematics with no thought at the time of their application.
So, maths can be used by all manner of people across every aspect of science and technology, including commerce and medicine, and of course in engineering. If you are interested in some aspect of mathematics, try to find where you could study the subject to the level you require.
As a mathematician you could find yourself working in or with almost any activity; computing, statistics, cryptography, crystallography, accountancy, logistics, finance, tax, investments, recoding, forensics, medicine, the list goes on and on. Everything can benefit from mathematics.
The Institute of Mathematics and its Applications, which is the UK's chartered professional body for mathematicians and one of the UK's learned societies for mathematics, provides much information that will help you discover the subject further and its potential for a career.