At the start of World War II, he joined Bethlehem Steel in Hingham, Massachusetts. He later became the first Jewish trade union delegate. After the war, Rosenberg borrowed $1,000 to add to his $1,500 in war bonds and used his knowledge of food distribution to open his first company "Industrial Luncheon Services", a company that delivered meals and coffee break snacks to factory workers on the outskirts of Boston. Rosenberg created his own catering vehicles, with sides that rose to reveal sandwiches and snacks stocked on stainless steel shelves, an early predecessor to mobile catering vehicle. Within a short time, he had 200 catering trucks, 25 in-plant outlets and a vending operation.