The low-temperature operations of diesel engines and aftertreatment systems have attracted increasing attention over the past decade due to the stringent diesel emission regulations and excessive tailpipe emissions at low temperatures. The removal of NOx emissions using selective catalytic reduction (SCR) systems during low-temperature operations remains a significant challenge. One of the popular techniques for alleviating this issue is to employ active thermal management via in-cylinder postinjection to promote aftertreatment system temperatures. Meanwhile, numerous studies have focused on ammonia coverage ratio controls with the aim to maintain high NOx conversion efficiency and low tailpipe ammonia slip. However, most of the active thermal management and SCR controls in the existing literatures were separately and conservatively designed, which can lead to higher cost of SCR operation than needed including diesel fuel consumption through active thermal management and urea solution consumption. The main purpose of this study is to design and coordinate active thermal management and SCR control using nonlinear model predictive control (NMPC) approach to minimize the total cost of SCR operation while obtaining high NOx conversion efficiency and low tailpipe ammonia slip. Simulation results demonstrate that, compared to the baseline control which consists of separated active thermal management and SCR control, the coordinated control is capable of reducing the total cost of SCR operation by 25.6% while maintaining the tailpipe NOx emissions and ammonia slip at comparable levels. Such an innovative coordinated control design concept shows its promise in achieving low tailpipe emissions during low-temperature operations in a cost-effective fashion.