O’Connor took over from Bill Michael earlier this year on an interim basis – after the former head was forced to step down following misconduct allegations.
However, the firm recently appointed Jon Holt as the new chief executive after receiving “overwhelming support” in a partnership vote, and he is set to hold the role until the end of September 2025.
It is understood that partners at KPMG were told on Wednesday that O’Connor was to exit the firm.
O’Connor first joined the group as its chief risk officer in 2018, before being promoted to the role of head of clients and markets in October 2020.
Speaking to the outlet, Bina Mehta, chair of KPMG UK, said: “Mary has made a significant contribution to KPMG, and we are hugely grateful for the commitment, focus and energy she’s brought to the firm.”
Accountancy Today has contacted KPMG for a comment.