As a youngster, Willenborg was first in the national rankings in both singles and doubles. He was nominated for two top All-American selections while studying psychology at UCLA . At the 1979 Universiade in Mexico City , he competed doubles for the USA and won the silver medal. After graduating in 1983, he became a professional tennis player.
In the course of his career he was able to win seven double titles. He reached his highest ranking in the tennis world rankings in 1984 with position 50 in singles and in 1988 with position 13 in doubles.
His best individual result in a Grand Slam tournament was reaching the second round at the French Open 1985, where he was defeated by Jimmy Connors , and a second round participation at the US Open 1980. In the doubles competition, he reached the quarterfinals of the US Open in 1988 and 1985 and 1987 the quarter-finals of the French Open. In mixed he was in the quarter-finals of the US Open in 1987 and in the quarter-finals of the French Open in 1989.