This paper introduces a novel approach to make polymeric thermal ground planes (TGPs), which consist of flat heat pipes covering large surfaces. TGP are typically made of copper, which is not adapted to recent applications such as in-mold electronics, wearable technologies or embedded systems. We introduce the polymeric TGP as a thermal management solution for these emerging technologies, where the flexibility and compactness are major advantages. On one hand, polymers bring flexibility and compatibility with 3D geometries. On the other hand, it gives access to high volume manufacturing processes such as molding that reduce the cost. Our novel fabrication process for polymeric TGPs allows assembly and sealing in a single step, with in-situ filling. The fabricated TGP has an effective thermal conductivity of 150 W/m K which is within the range of literature for polymeric TGPs and heat pipes as it varies from 100 to 800 W/m K. Thus, the work that we are presenting shows that we can make a polymeric TGP with a one-step fabrication process that results in a low thermal budget and low-cost process.