This study investigates the uniqueness and stability of solutions for infinite-dimensional systems with infinite delay, using almost-sectorial operators and specific conditions on the nonlinear terms. These findings are discussed in the context of recent progress on sectorial and almost-sectorial operators, expanding stability theory to include a wider range of time-varying operators and semilinear systems. The applicability of the theoretical findings is illustrated through several examples, including semilinear parabolic problems and abstract equations in Banach and Hölder spaces. These methods are demonstrated to be applicable to a wide range of infinite-dimensional systems with delay.