Although Reynolds was drafted in the 4th round of the amateur draft on June 5, 1979, by the San Diego Padres, he elected not to sign and joined the Cañada College Colts baseball team.
The following summer, on June 3, 1980, Reynolds was selected in the 1st round (2nd pick) of the amateur draft (Secondary Phase) by the Seattle Mariners. Reynolds signed with the Mariners after one season with Cañada College in Redwood City.
On June 1, 2013 Reynolds was inducted into the Cañada College Hall of Fame and was presented with the "Colts Lifetime Achievement Award".
Reynolds spent several seasons in the minor leagues, playing in Lynn, Massachusetts for the Lynn Sailors (AA) before being called up by the Mariners and making his major league debut on September 2, 1983. The following season he played AAA ball before being called up again in September 1984. The 1985 season was his official rookie season in Major League Baseball.
In 1991, Reynolds was a recipient of the Roberto Clemente Award, which is given annually to a Major League Baseball player selected for his character and charitable contributions to his community.
On October 26, 1992 Reynolds was granted free agency and signed with the Baltimore Orioles on December 11, 1992. After one season with the Orioles, he was again granted free agency on October 29, 1993. Reynolds signed with the San Diego Padres on January 28, 1994 before being traded to the California Angels on March 29, 1994 for Hilly Hathaway. The 1994 season was Reynolds' final season in the major leagues.
On July 24, 2006, Harold Reynolds was fired from ESPN. The ESPN spokeswoman confirmed that Reynolds "is no longer with the network" but did not give a reason for the departure. "Three people who work at ESPN and familiar with the case said the cause was a pattern of sexual harassment." Reynolds called this incident "a total misunderstanding" and that "I gave a woman a hug and I felt like it was misinterpreted."
It was announced on October 30, 2006, that Reynolds planned to sue ESPN after having tried "everything possible to handle this situation quietly behind the scenes," while stating that he is seeking the money owed to him under the remainder of his contract, including interest and lost earnings. The Smoking Gun obtained a copy of Reynolds' contract that was filed as part of the lawsuit. Reynolds' lawsuit is for $5 million, roughly equivalent to the value of the contract Reynolds signed that was scheduled to cover the 2006–2011 seasons.
ESPN settled the case in April 2008, giving Reynolds a seven figure settlement (Portland Tribune, April 29, 2008).
On June 11, 2007, Reynolds officially joined MLB.com as a baseball commentator. Reynolds settled his lawsuit with ESPN on April 16, 2008. Nine days later, Reynolds officially joined Mets pre-game and post-game coverage on SportsNet New York as a baseball commentator. Reynolds also worked with TBS on their Sunday baseball telecasts, as well as the 2008 MLB Playoffs.
Since its launch on January 1, 2009, Reynolds has been an analyst on MLB Network. Reynolds regularly appears on MLB Tonight, Quick Pitch, Diamond Demo and MLB Network's breaking news and special event coverage, including the All-Star Game, Postseason and World Series. He also stars in a new show alongside Brian Kenny titled MLB Now. The new series airs Monday thru Friday on MLB Network. Reynolds was nominated for a Sports Emmy Award for his work as a studio analyst on MLB Network in 2010 and 2011.
Reynolds became a member of the MLB on Fox pregame show in 2012, which at the time was being produced out of MLB Network's studios. Reynolds worked on Fox's pregame show for two years alongside Matt Vasgersian and Kevin Millar. With the retirement of Fox's lead analyst Tim McCarver following the 2013 season, Reynolds, along with Tom Verducci, was promoted to join Joe Buck on the network's top broadcast team.
Reynolds provides an in-game tutorial on how to hit, field, and pitch in the Triple Play Baseball and MVP Baseball series. He has also started an organization called HR Enterprises.