The native of Waltham, Massachusetts was taken 110th overall by the Pittsburgh Penguins in the 1987 NHL Entry Draft while he was in the midst of a fine career at Matignon High School. He then moved on to Boston University where he was named to the Hockey East First and Second All-Star Teams once each. McEachern committed to the U.S. national team in 1991-92 and registered 49 points in 57 exhibition games before taking part in the Albertville Olympics.

Following the Americans' fourth place finish in France, McEachern joined the Penguins in time to help them repeat as Stanley Cup champions. During his first season, in 1992-93, the talented forward scored 28 goals and helped Pittsburgh finish at the top of the regular season standings before the New York Islanders upset them in the playoffs.

In August 1993, McEachern was traded to the Los Angeles Kings for Marty McSorley. A few months later, he was sent back to the Penguins with Tomas Sandstrom for McSorley and Jim Paek. He continued to produce for the Pens but the club continued to re-tool in its attempt to return to the Stanley Cup finals. After scoring 24 goals for the Boston Bruins in 1995-96 and capturing gold at the 1996 World Cup, McEachern was dealt to the Ottawa Senators.

Upon joining Ottawa, the clever forward became a regular point producer and team leader on one of the league's top clubs. Beginning in 1997-98, McEachern began a run of four straight 20-goal seasons. His speed and scoring touch were important factors in the Sens reaching the 100-point mark in 1999 and 2001.

After six strong seasons in Ottawa, McEachern found a new home with the Atlanta Thrashers in 2002-03 and upon his arrival was a team leader both on and off the ice. During the summer of 2005, the Boston Bruins acquired the veteran forward. He would go on to appear in 28 games with the Bruins throughout their 2005-06 regular season and annouce his retirement from the game in the summer of 2006.