Conventional robot manipulators produce poor payload to weight ratio and limited manipulation ability, as a significant portion of available actuation force is used to balance their own weight. The same cause also limits the robot's operation capability in terms of acceleration and manipulation force. Such problems become more severe for modular and reconfigurable robots (MRRs) when they are expanded by adding predesigned modules. Static balancing with counterweights and external springs can greatly improve a robot's payload and manipulation capabilities, but require sophisticated mechanisms and restrict the working envelope of the robot. In this paper, an innovative spring-assisted MRR design and control framework is presented, which is developed based on a synergetic integration of robot control with a brake and an embedded spring at each modular joint. The spring is inserted between the brake and the motor shaft through a decoupling bearing. By activating the brake, static balancing can be established at any desirable position of each module and any configuration of the robot, allowing reinforced delicate operation in a neighborhood of the balanced configuration such as door opening, as well as spring-assisted lift of heavy payload. A distributed control method has been proposed to facilitate control of the spring-assisted MRRs, which does not rely on a priori dynamic models, and can suppress uncertainties caused by reconfigurations, eliminating the need to readjust control parameters of the lower modules when new modules are added or removed. Prototype modules have been developed, and the experimental results have confirmed the effectiveness of the proposed design and control.