Writing a Thesis in Theoretical and Mathematical Physics presents thesis writing as the construction of a clear, rigorous scientific argument - not merely a collection of equations, calculations, and references. The book guides postgraduate students, doctoral researchers, and early-career physicists in organizing ideas, motivating mathematical formalism, stating assumptions, presenting derivations, interpreting results, and connecting conclusions to research questions. A running example of coupled, time-dependent quadratic systems demonstrates these principles in practice. The approach balances mathematical precision with physical insight and applies broadly to quantum mechanics, field theory, gravitation, statistical mechanics, spectral theory, non-Hermitian systems, computational physics, and other areas of physics.