Frank L. Mingo, 49, chief executive of one of the nation's largest black-owned advertising agencies. Mingo spent more than 20 years in the advertising business, working at McCann-Erickson Inc. and the J. Walter Thompson agency before founding the Mingo Group in 1977. In 12 years, he built the firm from a small shop with four employees and $600,000 in annual billings to an agency with a staff of 50 and billings of $60 million. The agency's clients include Miller Brewing Co., General Motors, the Army and the Greyhound bus line. Mingo also was chairman of Muse Cordero Chen & Baca, a Los Angeles advertising agency. In Stamford, Conn., on Monday after a stroke.