ADP-ribosylation and ubiquitylation are key regulators of a wide variety of cellular processes, with the sophistication of their interplay becoming increasingly prominent, as illustrated by ADP-ribosylation-dependent ubiquitylation mediated by Legionella effectors. Recent biochemical studies have reported ester-linked ubiquitylation of ADP-ribose by DELTEX ligases, yet the modification sites of this dual-modification on cellular targets remain completely unknown. Here, our search for interactors of RNF114 revealed DNA damage-induced, PARP1/HPF1-dependent mono-ADPr on serine as a cellular target for ester-linked ubiquitylation. By developing a multifaceted proteomics strategy tailored to the chemical features of the composite modification and based on specific chemical disruption of its interaction with Di19-UIM, we identified unique ADP-ribosyl-linked serine ubiquitylation sites in human cells, including on histones and PARP1. We establish ADP-ribosyl-ubiquitylation as a cellular post-translational modification and propose that our tailored proteomic approach will reveal its widespread nature, along with additional conjugation chemistries, across diverse signaling pathways.