Since the early 1990s, Rosenthal has brokered the sales of historic properties in Los Angeles, many of which have been restored and revitalized as part of the redevelopment that has occurred in Downtown L.A. in recent years. At CB Richard Ellis, he founded the firm's urban redevelopment group.

Among Rosenthal's noteworthy deals was his representation of developer Tom Gilmore in the year 2000 in assembling a major housing conversion in the downtown market: the Old Bank District. In 2004, he managed the sale of two of downtown's best-known historic icons, the Eastern Columbia Building and the Oviatt Building.

Rosenthal, who joins Grubb & Ellis as a senior vice president, served as adviser in the creation of Downtown Property Owners, the first business improvement district in the State of California. He currently serves as an adviser to several property owner BIDs, and he is also an elected adviser to the Los Angeles City Redevelopment Agency.