This study experimentally and numerically investigates the hydrodynamic characteristics and heat transfer of developing and fully developed laminar liquid–liquid Taylor flows. The problem is conducted in circular mini-channels with different diameters subjected to a constant wall temperature boundary condition. An experimental setup is designed employing an open-loop water/oil two-phase nonboiling flow at mini-scale tubing sizes of 1.42, 1.52, and 1.65 mm. Two silicone oils with the dynamic viscosities of 1 and 5 cSt at several volumetric flow rates are used to establish segmented flow. The impacts of the channel diameter, viscosity, and flow rate ratio on the flow pattern, pressure drop, film thickness, and heat transfer rate are discussed. In good agreement with the literature, it is found that the pressure drop generated by the interface increases the total pressure loss by up to 200% compared to the single-phase flow. The results also explain how recirculating regions within the slugs influence the film region and the physics of backflow. Furthermore, introducing segmented water slugs significantly enhances the heat transfer rate as the dimensionless thermal length decreases. A significant relation between the recirculating regions and heat transfer has been demonstrated for the first time.