Mr Allan will take over from current CEO Bill Oliver, who is retiring from the business on 30 November.
Previously Mr Allan, 43, was Chief Executive of student accommodation developer Unite Group.
He joined the company in 1999 from KPMG LLP and has held a number of finance and commercial roles in the business, including Chief Financial Officer from 2003 to 2006.
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As a qualified Chartered Accountant and a member of the Royal Institution of Chartered Surveyors, he also serves as a Trustee Director on the non-executive board of Anchor Trust.
Bill Shannon, Chairman of St. Modwen, said: "Mark has an impressive track record and is highly regarded in the listed property sector. I am certain that under his leadership the Company will continue to build on its unrivalled regeneration expertise and proven ability to realise value from its extensive land bank.
Mr Allan will be succeeded at Unite by MD of Operations, Richard Smith.
He joined the company in 2010 and has been a member of the Group's Board since January 2012.
(LM)
