Krupnik was drafted with the 69th pick of the 2002 MLS SuperDraft by the MetroStars, but did not make the squad, and instead spent the 2002 season with the Des Moines Menace of the Premier Development League. He was signed by the MetroStars late in the season, again without seeing any first-team action, and was then released on free transfer to the Wilmington Hammerheads. After winning the 2003 championship with Wilmington, Krupnik was signed by Charleston Battery. During his time in Israel with the US Maccabiah team, Krupnik caught the eye of a scout from Maccabi Netanya. Manager Reuven Atar asked Krupnik if he could join the team for training but Krupnik had a contract in the US. It was his sister, Svetlana, who kept in touch with Reuven Atar and immediately after finishing his contract, Krupnik immigrated to Israel. Krupnik was attractive to Israeli club teams because he was eligible to receive Israeli citizenship and not count as a foreigner. During the 2005–06 season he got into a contract dispute with the management. After the conclusion of the season, ownership of the club changed hands, and the Israeli manager, Reuven Atar, who brought Krupnik to the club, was fired. Krupnik was expected to return to the United States. Talks ensued between him and Major League Soccer but nothing became final. On July 20, 2006 Reuven Atar was appointed as manager of newly promoted Israeli Premier League club, Maccabi Herzliya. His first action was to sign Leonard Krupnik to a contract with the club. Krupnik stayed with the club until the winter recess when he decided not to continue after Maccabi Herzliya would not agree to a new contract. After leaving Maccabi Herzliya, Krupnik joined Bnei Sakhnin for the second half of the 2006–07 season and helped the club gain promotion to the Israeli Premier League. The following season Krupnik helped the newly promoted side to a fourth-place finish and qualification to the Intertoto Cup. On June 17, 2008, Krupnik signed a contract with Maccabi Haifa for 3 years worth $360,000. During his first season with the club Krupnik was a fixture in central defense appearing in 30 league matches, and helped Maccabi Haifa clinch their 11th Israeli Premier League title. Krupnik returned the New York Red Bulls, known as the MetroStars during his first stint with the club, in July 2009. Krupnik made his MLS debut in a 4–0 loss to Colorado Rapids on July 25, 2009, and played in three games for the team before returning to Israel in November, when he was signed for a second stint with Maccabi Netanya.
International
In 2005, Lev Kirshner picked Krupnik for the United States squad for the 2005 Maccabiah Games, an international Jewish athletic event similar to the Olympics held in Israel every four years. Kirshner brought a talented squad with the most professional experience ever. The American team, which also had Jonathan Bornstein and Benny Feilhaber playing for it, reached the final but lost to the host Israeli team, which was composed of Israel's Under-18 national team players.
