The 22-year-old led the team out against London Irish in the Premiership Rugby Cup clash at cinch Stadium at Franklin's Gardens.
And as someone who has been at Saints since the age of 13, it meant so much to the Cambridge-born forward.
"I found out when they announced the squad in the team meeting, and I was really chuffed," Coles said.
"It's dream-come-true stuff to be captain of the club I've supported since I was a boy.
"I've grown up in the Academy and watched some of the players I ended up playing with on the day: the likes of Tom Collins.
"It was a really special moment for me and I made sure I took some time to reflect on that, before and after the game.
"It was just a shame we couldn't put a team performance together to cap off that moment that was very special for me."
So what kind of captain is Coles?
"I'm an organisational leader, I try to lead in my actions as well but I'm quite detailed and I like to bring people together and make sure they're all on the same page," he said.
"I'm probably slightly different to someone like Lewis Ludlam, who is very good at driving the emotional energy. I try to bring that as well but it's probably not my natural strength.
"I'm fairly good at getting people on the same page and generally making sure the team is cohesive.
"I'd like to think I'd did an okay job at that at the weekend because we rallied back from a tough start.
"It tested me in my first captaincy situation but the boys did well to get themselves back into a position to win the game.
"The experience showed through for them in the end."
Coles started the game at six, having grown up playing much of his rugby in the second row.
"I've done a bit of six in the past and the coaches have spoken to me about that being a position I can cover," he said.
"I enjoy playing there because it means you've got a bit more energy in the legs after scrums so I found myself making a few more tackles than I might normally because I was a bit fresher.
"I feel comfortable playing at six.
"I wouldn't say I have any particular preference between there and the second row - I'm just happy to get a game where I can!"
As a lock who is capable of playing at six, Coles looks up to Courtney Lawes, who has enjoyed a similar transition for club and country.
"I follow closely everything Courts does because he's a great role model for any aspiring lock or back row forward," Coles said.
"It seems to be quite common in the modern game that there are those lighter second rows who can jump between five and six.
"I want to be one of those and take on some of the elements of Courtney's game that he's doing so well."
Next up for Coles and Saints is a clash with Harlequins in a Premiership Rugby Cup encounter at the Twickenham Stoop on Friday night.
The black, green and gold go into the game on the back of a three-match losing streak after being beaten 36-26 by London Irish last weekend.
And Coles said: "Everyone's very aware of the fact that losing and winning become habits.
"As much as we had a young team out last week and we wanted to make sure we were performance-focused, we were also acutely aware that professional sport is about getting results.
"We need to go down to the Stoop and put ourselves in the best position to get a win to set ourselves up when the internationals come back going into the Premiership."