After graduating, Zimmerman remained at Hopkins for a season to coach the lacrosse program's B squad in 1977. He spent the following season as an assistant coach at Princeton University. From 1979 to 1982, he served as an assistant under Willie Scroggs at the University of North Carolina. While there, the Tar Heels won the national championship in 1981 and 1982.
Johns Hopkins
In 1983, he returned to his alma mater, and became the head coach in 1984 after the retirement of Henry Ciccarone. That year, he led the Blue Jays to a perfect 14–0 season and became the first head coach to win an NCAA championship in his first season. In 1985, Hopkins again won the national title while suffering one loss. The following year, the Blue Jays compiled a 10–2 record, and were eliminated in the semifinals in overtime by the eventual national champions, North Carolina. In 1987, Zimmerman led the team to a 10–3 record and a third title. Upon expiration of his contract, he left Hopkins after compiling a 6–5 record in 1990. The reason he cited was the pressure associated with the high-profile position. In total, Zimmerman's teams at Johns Hopkins posted a 73–15 record. Current Hopkins head coach Dave Pietramala spent his entire college playing career under Zimmerman, from 1986 to 1989. Former North Carolina and Hopkins head coach John Haus played for the Tar Heels under Zimmerman and then served as his assistant at Johns Hopkins. Zimmerman was the United States men's national lacrosse team's assistant coach for the 1986 World Lacrosse Championship.
UMBC
Zimmerman spent three years at Loyola as an assistant for the Greyhounds under Dave Cottle, before he was hired as head coach by the University of Maryland, Baltimore County. The Retrievers continued to struggle early in Zimmerman's tenure, and during his first three seasons compiled a 14–25 mark. After a 3–9 campaign in 1996, UMBC became the most improved team in the nation with a 9–3 record for the 1997 season. In 1998, the Retrievers delivered a major upset by beating, 12–8, then first-ranked Maryland, the eventual national championship runners-up. The victory helped UMBC secure its first ever berth in the NCAA tournament. In 1999, the Retrievers again made an appearance in the tournament. Zimmerman coached the South squad in the 2002 North-South Senior All-Star Game. In 2005, UMBC posted a 5–1 conference mark to achieve the America East regular season conference championship. The following year, the Retrievers defeated Binghamton and Albany in the America East tournament for the conference championship, and Zimmerman made his tenth appearance as a head coach in the NCAA tournament. He was named the 2006 America East Coach of the Year. In 2007, UMBC again advanced past Binghamton before falling to Albany in the America East championship game, and secured another berth in the NCAA tournament. There, they upset seventh-seeded Maryland in the first round for the school's first appearance in the NCAA quarterfinals. In 2008, UMBC again faced Albany in the America East championship game. Halfway through the second period, the Retrievers trailed 11–2, but rallied to edge their opponents, 14–13. Zimmerman was named the 2008 America East Coach of the Year. In June, he was considered the frontrunner to replace John Haus who had been fired as North Carolina's head coach, but Zimmerman removed his name from consideration. He said, "The people at UMBC have been good to me, and this is my home... these are my kids and I recruited them. I just didn't think this was the right time to move." In 2009, UMBC captured its fifth consecutive outright or shared America East regular season title. In the conference semifinals, the Retrievers edged Binghamton, 9–8. In the final, however, UMBC jumped out to an early lead over Albany and never relinquished it, and won, 15–7. Zimmerman was named the 2009 America East Coach of the Year. In the first round of the NCAA tournament, the Retrievers faced sixth-seeded North Carolina. UMBC entered the game with the number-one ranked midfield in the nation, but it was lacking in depth, and the first-string was thus forced to play most of the game. The Retrievers led 8–6 at halftime, but North Carolina assumed control during the third period and went on to win, 15–13. While coach at UMBC, Zimmerman has traveled to Japan, Argentina, and the United Kingdom in order to promote the sport of lacrosse. In 2002, 2004, and 2006, UMBC hosted fall scrimmages against teams from Keio University. In the summer of 2005, the Retrievers traveled to Japan. Regarding the possibility of the sport being a future Olympic event, Zimmerman said, "It’s incumbent upon us to introduce and develop the game when opportunities occur in order to reach that goal." The Greater Baltimore Chapter of the United States Lacrosse Hall of Fame inducted Zimmerman on January 25, 2003.
