He is succeeded by Frank Keenan who joins from Play.com, where he was a category director. Marshalls said it hopes to tap into his expertise on websites and use him to push the firm towards new target audiences.
"Marshalls is growing fast," said finance director Ken Evans. "Martin is an entrepreneur and a small-business set-up person who'd taken Marshalls as far as he could. It's an amicable parting."
Harvey was "absolutely pivotal" to the growth of Marshalls and Unwins Seeds and for helping steer Marshalls to becoming an ú8m-turnover operation, said Evans. His former colleague will be available should Marshalls want "help or guidance".
Keenan is a big player in the online business, where Marshalls wants to make big inroads, said Evans: "We are still very much a catalogue company but the world is changing and we need to be a better online business.
"Frank understands the online consumer and we have to start targeting a younger audience who want products just in time, not in three months, which is our traditional offering."
Evans said the firm is looking at areas such as TV shopping and focusing adverts in women's interest and leisure magazines to target readers of a similar demographic - "more Saga Magazine than a kids' gaming title".
Harvey started at Levington in 1973, where he launched the Gro-Bag.