After passing a physical and agreeing to a signing bonus of $1.6 million, according to a source, Kelly took batting practice and fielded grounders during the Cardinals' pregame work.
With Matheny, hitting coach Mark McGwire, and the rest of the St. Louis squad on hand watching his swing, Kelly stayed calm and blended in while hitting liners to the gaps and a handful of balls over the wall.
"He beat the over/under," Matheny joked. "He went his whole first round hitting line drives up the middle, so you could tell he's got some maturity. Most kids come out here and just start launching them. They take a huge swing and walk out of there with their head hanging low."
After mulling over a difficult decision between pursuing a career professionally or attending the University of Oregon on scholarship, Kelly decided earlier this week to sign with the Cardinals.
"I took everything into account: The money, the opportunity to go to college and play for coach [George] Horton and the Ducks," Kelly said. "This was just a very, very tough decision. The way it turned out, I'm happy with it."
The Cardinals have a little more than $9.1 million to spend on their first 14 picks as a result of rules in the new Collective Bargaining Agreement.
As the 86th overall pick in the Draft, Kelly had a recommended signing bonus of $574,300. But by drafting other players in the first 10 rounds who could be signed for below the recommended slot, the organization was able to get aggressive in offering Kelly much more.
"I think as you see from Day 1, how we allocated our money, that allowed us the flexibility to then go after somebody like Carson," Cardinals scouting director Dan Kantrovitz said. "Carson was top on our list of guys to pursue in that category. It helped make our Draft to land somebody like him on Day 2 in the second round. We valued him as a first-round talent."
Kelly said he had a "wait-and-see-what-happens" mentality throughout the whole Draft process, and added that the negotiations with the Cardinals went smoothly.
"I was very optimistic with the process, that's just the way it went," Kelly said. "It was a long process, but in the end we came to an agreement, and I'm just happy to be here."
Kelly will join the Cardinals' rookie affiliate in Johnson City (Tenn.), which begins play on Tuesday, and will play third base. Kelly has not played a game since his Westview High School team was knocked out of the Oregon state playoffs about two weeks ago.
Kelly took ground balls with Cardinals second baseman Tyler Greene on Friday, who gave Kelly tips and advice on how to develop a plan to make it through the grind of the Minor Leagues.
While Kelly said he would like to speed through the farm system and join the Cardinals as soon as possible, the 17-year-old is keeping the process in perspective and will take the journey one step at a time.
"I'm just going to keep working," Kelly said. "Hard work pays off, and we'll see what happens. But I'm going to continue to keep working and hope all goes well."