During his tenure as chairman of Colonial Williamsburg, Winthrop was a frequent visitor at the foundation's Carter's Grove Plantation eight miles away in James City County, Virginia. He is credited with helping develop a plan with Gussie Busch in the early 1970s to turn a portion of the large tract of undeveloped land between the two points into the massive Anheuser-Busch (AB) investment there, which included building a large brewery, the Busch Gardens Williamsburg theme park, the Kingsmill planned resort community, and McLaws Circle, an office park. AB and related entities from that development plan now are the source of the area's largest employment base, surpassing both Colonial Williamsburg and the local military bases.