During his sophomore season in the WHL, 2003–04, MacArthur helped lead the Tigers to the Ed Chynoweth Cup as League champions and a berth in the 2004 Memorial Cup. MacArthur was the fourth-leading scorer in the tournament with one goal and four assists and was named to the Memorial Cup All-Star Team. The Tigers, however, lost in the semifinal of the tournament to the Kelowna Rockets. MacArthur played in 58 games during the 2004–05 season with the Tigers and recorded 74 points, while serving as one of the club's alternate captains. After the season, he was named to the WHL's Eastern Conference First All-Star Team. MacArthur was remarkably consistent during his WHL career, scoring 75, 75 and 74 points in his three WHL seasons.