In 1981–82, Fergus surprisingly made a strong Boston team at the age of just 19, and turned in a fine rookie season with 15 goals and 39 points in 61 games.

With Pederson injured for much of the 1984–85 campaign, Fergus spent most of the season on Boston's top line, and set career highs with 30 goals and 73 points.

He also made his international debut with the United States national team in the 1985 Ice Hockey World Championship tournament in Prague after the Bruins were knocked out in the first round of the Stanley Cup playoffs.

However, with Pederson's return the following year along with Ken Linseman Boston was left with three scoring-line centers, and Fergus was dealt to the Toronto Maple Leafs for Bill Derlago.

He returned to form in 1988–89, registering a career high 45 assists and 67 points, although he drew criticism for his defensive play and his league-worst -38 rating.