The bookmaker noted the reasons for Marsh's decision were due to the current unprecedented economic circumstances around the coronavirus pandemic, having been at DS Smith for seven years.
Roger Devlin, chairman of William Hill, said the company has restarted the search for a new chief financial officer, with current CFO Ruth Prior continuing to serve out her 12-month notice period.
She gave notice in January that she was leaving.
Miles Roberts, DS Smith’s chief executive, said: “We are pleased that Adrian will be remaining with DS Smith, providing continuity and stability in this very important time.”
Analysts previously said Marsh was a "pivotal part" in the packaging group's growth as it was promoted to the FTSE 100 in 2017.