Pyrolysis of waste plastics to recycle valuable hydrocarbons represents an attractive technology for reducing waste and providing feedstocks for petrochemical products and fuels. Via the simultaneous processing of heavy petroleum residue fractions, synergies can be harnessed by converting bottom-of-the-barrel refining products into lighter fractions with higher value while improving processability of plastic waste materials. To investigate the effect of reactor pressure, a continuous laboratory co-pyrolysis plant was operated. The setup consisted of two consecutive tubular zones to convert a mixture of LDPE and a heavy petroleum residue to a final temperature of 450 °C at different pressures between 2 and 10 bar. The products were evaluated regarding obtained mass yields and their boiling range. Gaseous and liquid products increased with enhanced pressure, resulting in nearly tripled gas and light liquid formation, whereas more unconverted feed was consumed. Because the reactor pressure also affects the residence time by suppressing evaporation, which subsequently varies between 360 and 440 s, further investigations considering the dependence of product yields on the residence time over a range of 280 to 480 s were necessary. The comparison resulted in the conclusion that the enhancing effect of increased reactor pressure is not only caused by a retention time elongation in the hot reactor zone. Other physical effects also play a role, such as promoted heat transmission and a direct intervention of reactor pressure with the chemical reactions. In the tested range, an enhancing effect of higher reactor pressures on the cracking of the reaction mixture was observed. These novel experimental results indicate, that conversion toward lighter cracking products can be increased by pressure adjustments and highlights that the pressure should be included in process optimizations.