Abstract: Guide track assemblies for upward acting doors include substantially linear vertical and horizontal track sections and curvilinear transition sections formed as portions of each of the vertical and horizontal linear sections. The track assemblies are supported, respectively, by mounting brackets which also support a counterbalance mechanism for the door. The curvilinear transition track sections are formed integral with the vertical and horizontal track sections and may have a constant or elliptical radius of curvature, the latter to provide clearance for the guide track assemblies in low headroom installations. The mounting brackets are formed of stamped metal plates which are folded to provide a first flange for attaching the mounting bracket to a structural wall, a support part including a flag portion for connection to the guide track sections and a second flange for connection to a door counterbalance mechanism.