Lemieux (34-2) dominated the bout, flooring N'Dam (31-2) once in the second round, twice in the fifth and again with a left hook in the seventh to win the title.
But each time he was knocked down, N'Dam got back up and dished out punishment of his own to Lemieux, who said later that his 31-year-old opponent didn't hurt him but certainly earned his respect.
"Not only did he get up, but he got up smiling," said Lemieux. "I was surprised.
"He's a solid boxer, the real deal. He wanted to be champion but I wanted it more."
Two ringside judges scored it 115-109 and the other 114-110, all for 26-year-old Lemieux.
Lemieux joined WBC light heavyweight champion Adonis Stevenson as Canadians holding world boxing titles.
After the bout, Lemieux's promoter Oscar de la Hoya said N'Dam "deserves another opportunity to fight a world champion."
"David Lemieux is a great boxer," said N'Dam. "After he hit me in the second round I had to finish the fight at only 60 per cent of my capacities.
"He was better than me tonight, but I think that other than the ones when I was knocked down, I won the rounds."
Lemieux's impressive performance in a fight broadcast in the U.S. on the Fox network may earn him some high profile bouts. There is already talk of a clash with WBA champion Gennady Golovkin, or a bout with the winner of a proposed match in November between WBC champ Miguel Cotto and Mexican star Saul (Canelo) Alvarez.
In the co-feature, Dierry Jean (29-1) of Montreal retained his North American lightweight belt with a 12-round unanimous decision over Jerry Belmontes (19-7).