Chalmers is the first Aussie man to win the 100m freestyle since 1968, and only the fourth to ever do so.
Speaking after the race, the teen said he couldn’t have been happier.
"It's awesome. I didn't have any more to give," he said. "But there's mixed emotions. It's hard winning and him (McEvoy) not swimming his best."
"It's definitely still sinking in I have actually won... I faded towards the end there especially when I got under the flags, I was worried I glided too long at that finish."
Cameron McEvoy was touted to be Australia’s biggest chance on the race, but the 22-year-old finished seventh.
"It is baffling to me, particularly with my analytical mind," McEvoy, 22, said after the race. "Hopefully, I can get back and pinpoint what went wrong. Right now, it's hard to pinpoint anything specifically."
However, in true Aussie spirit, he managed to put his own disappointment aside to congratulate his Rio roommate.
"That's all I can say about myself. The rest of Australia can get excited about an 18-year-old's start to his career. He's an Olympic champion at 18. When I dwell on it, it is tough and I expect it to sink in over the next few weeks. Funnily enough, it doesn't feel as gruesome and raw as coming second at worlds last year, which is odd because this is much bigger. But Kyle's success and happiness, I can feel that. I'm riding the wave that he's won."
McEvoyy also took to Instagram to share a heartwarming photo, with his young teammate.
"I won't ever forget this moment here in the pool. My embrace in full support of Kyle, and Kyle's embrace in full support of me. Although there were 15,000 people watching and another several hundred million watching on TV, this moment was just between the two of us. Complete respect: this is what sport is all about. Thank you. #OurTeam @dolphinsaus @ausolympicteam"