Marler made his 286th and last appearance for Quins just four weeks after announcing his England career was over.
The 34-year-old left the field to a standing ovation in the 48th minute but Bristol spoiled the party as they moved to the top of the Premiership table.
Six tries were shared in the first half as Kalaveti Ravouvou, Gabriel Ibitoye and Kieran Marmion crossed for the Bears, while Alex Dombrandt, Cadan Murley and Luke Northmore replied for the home side.
The game was still in the balance when Marler, one of rugby's most colourful characters, disappeared down the tunnel, but Bristol quickly galloped out of sight in a dominant third quarter decorated by three outstanding tries.
Bill Mata, Bernhard Janse van Rensberg and Santiago Grondona were the scorers as Pat Lam's men tore the Premiership's second tightest defence to shreds.
Newcastle climbed off the foot of the table with a hard-fought 17-12 victory over Saracens.
Tobias Elliott scored the first try of the evening for Saracens, but Newcastle responded with two of their own after Alex Hearle and Adam Radwan both crossed for the hosts to lead 17-5 at half-time.
A quieter second half saw Sarries begin to push into Newcastle territory and Tom Willis scored a try in the final minutes, but a fantastic defensive display saw Falcons secure just their second win in 28 Premiership matches.