With the AFC West and their playoff seed clinched, the Kansas City Chiefs had the luxury of sitting starting quarterback Alex Smith and playing Mahomes in Week 17. With the score tied at 24 and 2:45 remaining on the clock, Mahomes got his first chance to shine.
Mahomes, completing four passes for 52 yards and converting twice on third down, led the Chiefs to a walk-off field goal and a 27-24 win.
The Chiefs were leaning toward trading Smith and installing Mahomes by this season even before they got to Denver that day. But the way Mahomes took care of business, both during the game and the practice week leading up to it, confirmed it.
"I like the way he went about business," Reid said. "We were able to take him and lead him into that game throughout the whole week. ... What that did was it allowed you to get a feel for him for down the road, whenever that time was, and I liked what I saw and felt there. I think he handled things very well. Prepared to the 'T' on that, handled himself very well.
"I came out of that going 'You know what? This kid is ready to go. He's ready to go ahead and lead.'"
The Chiefs pulled a rare double that week, working on two games at once. Smith studied playoff game plans with then-offensive coordinator Matt Nagy and assistant coach Brad Childress, while Mahomes prepared for the Broncos with coach Andy Reid and assistant Mike Kafka.
That's right, Reid spent his time with Mahomes on a game that was meaningless to the Chiefs in the standings, but not to their future. It was important for Reid to get his first-round pick off to a good start in the first start of his career.
The Chiefs couldn't have hoped for a better situation, a chance for their prized rookie to prove himself in crunch time. And in that moment, Mahomes' demeanor gave them confidence things would turn out all right.
"He just had this calmness about him and that seems to be natural for him," tackle Mitchell Schwartz said. "He didn't get intimidated by the moment, and that's something you want to see on the road in Denver.
"In practice, when he's on the scout team, he can run around and chuck it and not really have to worry about things. The game is really when you get to know how someone can handle pressure situations. It was good to see. It was a fun game, and it turned out well."
Now in Week 4 of 2018, Mahomes and the Chiefs return to Denver for the first time since that start, this time on Monday Night Football. He's no longer a rookie trying out his talents for the first time. Mahomes has been the sensation of the early season, throwing for 13 touchdowns without an interception, and the Chiefs are one of the NFL's three remaining unbeaten teams.
But nothing was certain last season heading into Denver. Reid and Kafka tailored the game plan to Mahomes' strengths. They operated from the shotgun, as Mahomes had done almost exclusively in college, on about half of their plays.
"I was able to ... talk to him and see what he liked and felt most comfortable with," Reid said. "For that game, we molded that game plan around him. Mike Kafka spent a lot of time with him when everybody else was kind of working on the playoff game. We were able to get in there and kind of mold it with what we had in the package at that time, mold it around him."
They made little effort to provide him with extra protection, using more than five blockers on seven of his 35 pass attempts.
Mahomes didn't get off to a great start. He threw incompletions on three of his first four passes, but the Chiefs took the lead anyway on their first drive on Kareem Hunt's 35-yard touchdown run. Mahomes threw his only interception on the next possession, firing too high for De'Anthony Thomas. Mahomes later acknowledged the overthrow was a result of being too excited.
He settled down after that and finished 22-of-35 for 284 yards.
"He helped call [plays] to my strengths, and we had success and came out of there with the win," Mahomes said.
He showed his talent for making difficult passes a couple of times. First he thew off his back foot because of pressure and connected with Albert Wilson between two defenders for 19 years. Later, while retreating because of pressure, he hit Demarcus Robinson for 12 yards between three Broncos.
"I was really impressed with the way he moved out of the pocket," center Mitch Morse said. "I didn't know he had those legs on him. He was able to throw the ball in those contorted, weird ways, [and that] was amazing. He's a playmaker. That was apparent. It was pretty special to watch him go out there and put drives together."
If those throws were Mahomes' signature plays of the game, the ending was the most important sequence. Mahomes, after being removed from the game in the fourth quarter with the Chiefs ahead 24-10, re-entered for the final drive.
"His ability after something didn't go right -- he threw an interception early -- he bounced right back and led us to a field goal," said Kafka, who replaced Nagy as quarterbacks coach this year. "Then he also came back in the two-minute drill and then won the game. Throughout the entire game, he kept his poise. He was very demonstrative about what he wanted to do. He made a ton of checks at the line. He operated and performed and played to win the game.
"It goes to his competitive nature. Honestly, when he came back in we all knew we were going to go back down and win, either score a touchdown or kick the game-winning field goal."
Mahomes won over Reid and the assistants that day. Maybe more important was that he won over the locker room. He was about to replace a popular veteran in Smith.
A positive first impression went a long way.
"That kind of solidified his status and his ability to lead a team in the future," Schwartz said. "Things were smooth the whole week in practice and you appreciate that. That tells you he was doing stuff Monday and Tuesday to prepare for the game.
"It just felt natural. It didn't feel weird without Alex in there. It just felt like there was a really good quarterback with you in the huddle, and things were going to turn out well."