Brownrigg returned to Britain and took up medicine with an established doctor called Richard Senhouse in Whitehaven. Senhouse died soon after, making Brownrigg the principal doctor in the area for many years to come. His casebook for 1737-1742 survives and was recently transcribed. It contains descriptions of his patients and remedies and some of the earliest English references to puerperal fever.
In 1741, Brownrigg married Mary Spedding. Marys father and uncle ran the collieries for James Lowther, whose family had developed Whitehaven into a major seaport. This increased Williams local influence and also promoted his interest in the health and welfare of the miners.
Later in 1771, with the threat of an epidemic from Europe, Brownrigg who had studied the subject from outbreaks of typhus at Whitehaven, published a paper "Considerations on the means of pestilential contagion, and of Eradicating it in Infected Places."