Targeted inhibition of DNA damage response proteins has received significant clinical attention owing to the success of PARP inhibitors. Due to the loss of the G1/S checkpoint, cancer cells are reliant on the G2/M checkpoint to cope with elevated DNA replication stress. We previously demonstrated a single induction of 8-oxo-guanine at telomeres in cancer cells was sufficient to induce replication stress, but was well tolerated at the cellular level. Here, we found inhibition of ATR, Chk1, or Wee1 after induction of telomere oxidative stress significantly induced genome instability and reduced cell viability. This occurred at doses markedly less than those required to increase instability in non-cancer cells. We determined the mechanism of this instability is due to cells progressing through S-phase with telomere damage and exiting G2-phase prematurely, prolonging their mitosis. This study demonstrates targeted oxidative base damage at telomeres can enhance the therapeutic efficacy of ATR inhibition in cancer.