McGregor was expected to face current champion Aldo on 11 July 2015 at UFC 189 for the undisputed UFC Featherweight Championship. However, in the days leading up to the fight, Aldo pulled out of the bout because of a rib injury he had sustained in training. McGregor remained on the card and faced UFC ranked No. 1 contender Chad Mendes for the interim championship.
McGregor walked out to the octagon in front of more than sixteen thousand attendees as Sinéad O'Connor performed a rendition of "Foggy Dew". McGregor was taken down several times by the All-American wrestler in the first round, but was able to score a second-round TKO after getting back up to his feet, knocking down Mendes, and finishing with ground and pound with three seconds left, winning the UFC interim Featherweight Championship. This forced upon Aldo a mandatory championship bout with the interim title holder who was now Conor McGregor.
The victory extended McGregor's Performance of the Night Award streak to four, while the card drew a record $7.2 million gate and was a new United States record for mixed martial arts. The weigh-ins for the event also proved groundbreaking, with capacity being reached almost thirty minutes before its start time. The 11,500 attendance dwarfed the previous record of 8,000 set by UFC 148: Silva vs. Sonnen II. Later McGregor revealed that during the fight camp for UFC 189 he tore 80% of his ACL.
Shortly after UFC 189, it was announced that McGregor would be coaching against Urijah Faber in the following season of The Ultimate Fighter. In "U.S. vs. Europe", the twenty-second installment of the reality series, it was confirmed that the coaches would not fight against each other at the show's conclusion, unlike the majority of previous seasons.
On 12 December 2015, at UFC 194, McGregor defeated Aldo by knockout, thereby unifying the title in the featherweight division and became the only European male to win a UFC Championship by beating the champion. Before the opening of the first round, both Aldo and McGregor refused to touch gloves and began fighting immediately. As the fight started, McGregor took control of the center of the octagon. When Aldo got within his range, McGregor threw a straight left hand to keep him away, but missed. McGregor then followed with a leg kick that connected. As Aldo surged forward to connect with a right-left punch combination, McGregor slipped the first punch then connected with a left cross that landed on Aldo's chin and rendered him unconscious. Both left hand strikes connected, but McGregor's landed first. As Aldo fell to the ground, McGregor connected with two hammer fists before the referee stepped in to stop the fight. Within a minute, Aldo regained consciousness and was on his feet. The bout lasted 13 seconds, the fastest finish in any UFC title fight. "Again, nobody can take that left-hand shot," said McGregor in the post-fight interview. "Aldo's powerful and fast. But precision beats power, and timing beats speed. And that’s what you saw there."