Willenborg was as a teenager, both in single as well as double the national ranking first. He was elected twice to the best selection of All-American while studying psychology at UCLA. In the Universiade 1979 in Mexico City, he went to double for the U.S. and won the silver medal. After graduating in 1983, he was a professional tennis player.
Throughout his career, he was able to win seven doubles titles. Its highest listing in the tennis world ranking he reached in 1984 with position 50 in the singles and in 1988 with position 13 in doubles.
His best individual result at a Grand Slam tournament was reaching the second round at the French Open in 1985, where he was Jimmy Connors inferior and a second-round participation in the U.S. Open 1980. In the doubles competition, he reached the quarterfinals of the U.S. Open in 1988 and 1985, In 1987, the quarterfinals of the French Open. Mixed In 1987 he stood in the quarterfinals of the U.S. Open in 1989 and in the quarterfinals of the French Open.
After his tennis career Willenborg worked as a financial advisor for Merrill Lynch and later an executive with Morgan Stanley Smith Barney.