Lawler (26-10) battered MacDonald for much of the final four rounds, leaving the Quesnel, B.C. native with a badly damaged nose. Lawler finally ended it with a big left hand directly into the face of MacDonald, who rolled onto his back in agony before Lawler finished him 60 seconds into the fifth.
Lawler also was staggered repeatedly by MacDonald, and he barely made it to the bell in the third round of an occasionally jaw-dropping brawl. Both fighters bled profusely in the final rounds, but Lawler persevered for his first successful title defence and his second victory over MacDonald (18-3).
"I showed everyone I'm a true fighter," Lawler said. "I come to fight no matter what happens."
The welterweights put on an impressive show in the penultimate fight on the UFC 189 card headlined by Conor McGregor's interim featherweight bout against Chad Mendes.
After a tentative first round, Lawler caught MacDonald repeatedly with precise strikes to the face in the second, badly damaging MacDonald's nose and opening his face. Lawler controlled the third round as well before MacDonald hurt him with a head kick in the final minute, pounding him against the cage to the bell.
MacDonald stayed aggressive in the fourth and battered Lawler around the cage, but Lawler landed several big shots of his own. The fighters stayed at the centre of the ring talking trash after the bell — and early in the fifth, Lawler finally ended it with one left hand.
"That was the cumulation of a beatdown," Lawler said.
Lawler hadn't fought since he seized the 170-pound title from Johny Hendricks last December with a split-decision victory that capped a remarkable mid-career renaissance.
Once a vaunted teenage prospect, Lawler made his UFC debut in 2002, but slid out of the promotion for nearly 8 1/2 years. He won six of his seven fights after returning, including three in seven months after losing his first bout with Hendricks.
Lawler also beat MacDonald by split decision in November 2013, slowing the progress off the exciting prospect from British Columbia with heavy hands and a quirky personality. His narrow loss to Lawler was his only defeat in nine fights since 2010, and MacDonald rebounded with three straight victories to earn his first welterweight title shot.
MacDonald has been nearly unbeatable since moving to Montreal to train at Georges St. Pierre's gym, and he was eager to follow in the long-reigning welterweight champion's footsteps.
MacDonald hoped to become the third Canadian champion in UFC history, joining St. Pierre and Carlos Newton, who held the same belt for six months in 2001.
Lawler and MacDonald had few of the normal responsibilities to promote a title fight, thanks to McGregor's enormous personality on top of the UFC 189 card. The relative lack of attention suited both welterweights perfectly in a civil, complimentary buildup to their bout.