Flat studied at Southern Illinois University Edwardsville, for which he took up in singles and in doubles. Between 1981 and 1983 he won the NCAA Division II title, he also stood in 1983 in the doubles final of the Division I. At his side was already at that time Robert Seguso, who would become his long-term partner.
Flat was one of the best men's doubles players in the second half of the 1980s and first half of the 1990s. 1985 and 1993, he won the doubles competition of the U.S. Open, 1987 and 1988 in Wimbledon. Except at the U.S. Open in 1993, where he was victorious at the side of Rick Leach, he won all the Grand Slam titles with Robert Seguso. He also won the 1986 mixed- competition of the French Open and Wimbledon, both on the side of Kathy Jordan.
In the individual he was not nearly as achieving great success as a double. He could not reach an ATP final. His biggest success in the single was the first knockout round at the U.S. Open in 1987; his best finish in the tennis world ranking he reached in 1985 with 56 position
Flat graduated 1985-1991 13 double encounters for the U.S. Davis Cup team, of which he has won eleven. At the Olympic Games in Seoul in 1988, he joined with Robert Seguso for the United States and won the Olympic gold medal.
Since his retirement from professional sports Flat operates at Vanderbilt University as a tennis coach. He is married since 1986 with former fashion model Sandra Freeman, with whom he has four children.