A JMock developer uses the JMock library to create mock objects and carry out unit testing in Java applications. They design mock interfaces to simulate the behavior of real objects, isolating the component under test to ensure it functions correctly independent of external factors. They also define expectations, specifying the order and arguments of the interactions. JMock developers use these mock objects to test methods, exceptions, and return values. Their role is crucial in ensuring the reliability of the software, reducing bugs, and speeding up the development process by allowing testing before the implementation of dependent objects.