Seguin drew Boston even at 1 with his second preseason goal 4:17 into the third off a rebound in front of Carey Price. Clark put the Bruins ahead with his first goal at 13:15.
Michael Cammalleri gave the Canadiens a 1-0 lead with his third goal 15:33 into the second. Cammalleri, who didn't play in Montreal's 7-3 loss to Boston on Sunday in Halifax, Nova Scotia, had a pair of power-play goals in the Canadiens' 3-2 loss to Ottawa on Saturday.
Tuukka Rask stopped 30 shots for the reigning Stanley Cup champion Bruins (3-0-1). Price went the distance and made 30 saves for Montreal (1-5).
Boston beat Montreal in the first round of the postseason last spring, one of the Bruins' record three seven-game playoff series wins on the way to their first championship since 1972.