Matt Cullen has been working on downtown Detroit’s revival since the Carter administration. His real-estate career—beginning with General Motors Co. in 1979 when that company dominated—tracks Motown’s long demise, and its recent resuscitation.

In September, he became chief executive of Bedrock, a development company created by billionaire Dan Gilbert. As the coronavirus pandemic spreads, Mr. Cullen must bolster momentum Detroit has gained since its 2013 bankruptcy.

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