In the spring of 1987, Kerr spent his last semester of college as an exchange student in England. While in England, he played with Isthmian League club Harrow Borough towards the end of 1986–1987 season reportedly attracting the attention of several English Football League professional clubs. In June 1987, the Tacoma Stars of the Major Indoor Soccer League selected Kerr in the second round of the MISL draft. He declined to sign with the Stars. In the summer of 1987, he returned to England and signed with Portsmouth (then newly promoted to the First Division) on the recommendation of Peter Osgood. Making his First Division debut for the club away at Oxford United in a 4–2 defeat on August 15, 1987. During the 1987–88 season, Kerr made a first team total of four league and two cup game appearances together with a 3-month loan spell at then Fourth Division club, Peterborough United. During one of those first team appearances for Portsmouth, Kerr was to make English Football League history when on September 19, 1987 while on the field of play as a replacement in a First Division away fixture at Watford, he became the first substitute to be likewise, substituted. The English Football League rule change that increased the number of player substitutions during a game from one to two per side having been introduced at the start of the previous 1986–87 season. Kerr made his final First Division appearance for Portsmouth as a substitute in a 4–1 away defeat to Luton Town on March 29, 1988. Following his release from Portsmouth, Kerr returned to the United States and signed with the Washington Stars of the American Soccer League (and coached at the time by his father, John Kerr.Sr) in March 1988. He remained with the Stars for three seasons, the last in the American Professional Soccer League. While playing for the Stars during the summer, Kerr returned to Europe with English club Wycombe Wanderers, then playing in the GM Vauxhall Conference League, for the 1988–89 season. Making a total of 48 appearances and scoring 22 times in league and cup games for the Buckinghamshire club on its way to finishing in fourth position behind eventual Conference champions of that season, Maidstone United. Following his involvement with the U.S. squad as it prepared for the 1990 FIFA World Cup, Kerr had spells during 1990–91 with French Third Division side Boulogne-Sur-Mer and Northern Ireland club Linfield. In 1991, he returned to Canada to play one season with the Hamilton Steelers of the Canadian Soccer League. In October 1991, Kerr signed with the San Diego Sockers of the Major Soccer League. Kerr established himself as a regular on the team which went on to win the 1992 MSL championship. Following the collapse of the MSL during the summer of 1992, Kerr briefly spent time as an assistant coach with the Duke Blue Devils men's soccer team before returning to England, joining Isthmian League club Chertsey Town in the fall of 1992. He then moved to Football League club Millwall signing as a free agent on February 26, 1993 before temporary returning to the U.S. during the off-season summer break to continue his assistant coaching duties at Duke. Kerr went on to make a total of 40 first team appearances for Millwall in league and cup games during the 1993–94 and 1994–95 seasons, scoring 7 goals in the process. Towards the end of his time with Millwall, he also had a short on loan period with Walsall. In May 1995, Millwall gave Kerr a free transfer to the San Diego Sockers, however, he did not play for the Sockers. On February 8, 1996, the Dallas Burn selected Kerr in the ninth round (eighty-third overall) on the 1996 MLS Inaugural Player Draft. On June 27, 1996, Kerr was part of the first in-season trade in MLS history when the Burn dealt him to the New England Revolution for Zak Ibsen. He was later loaned out to the Connecticut Wolves. In 1998, Kerr was appointed player-coach with the Worcester Wildfire of the USL A-League, the following year the club was renamed the Boston Bulldogs after a change of ownership. In April 1999, Kerr returned on loan to the Revolution when several players on the team were ruled out because of injury. However, Kerr did not play during his loan period.
While at Duke, Kerr began his international career having become a naturalized U.S. citizen. He made his international debut in a scoreless draw versus Ecuador on 30 November 1984 in New York City. He soon became a regular player on the team and saw considerable playing time until 1988. From then until 1995, however, he failed to earn any more caps. As a result, he missed out on both the 1990 and 1994 World Cups. In 1995 he returned to the U.S. team and was included on the squad that surprisingly reached the semi-finals at the 1995 Copa America. He finished his national team career with 16 appearances and 2 goals.