‘You knew he would do big things.’ Joe Mazzulla’s Rhode Island connections aren’t surprised by his success.

On Monday night, the Celtics defeated the Mavericks in Game 5 of the NBA Finals to clinch the franchise’s NBA-record 18th title, which fans were eager for after a 16-year championship dry spell.

Rhode Islanders attribute part of the team’s success to the basketball legacy Mazzulla began at home. Mazzulla grew up in Johnston, R.I., just 60 miles south of the arena in which he made history this week.

The small town has a wealth of spirit when it comes to cheering on its own. Lines were out the door at D. Palmieri’s Bakery on game day, with locals celebrating a Celtics special of 10 percent off pizza strips.

Eric Palmieri, baker and pizza artist at the shop, said Mazzulla represents Johnston “incredibly well.” Mazzulla grew up down the street from the bakery and is a big fan of the pizza strips, he said.

“When anybody from Rhode Island has success or makes a name for themselves, everyone feels the excitement along with them,” Palmieri said. “The state is so small, we’re like one big hometown here.”

Gomes met Mazzulla when he was a bright-eyed 14-year-old with his sights set on starting high school at Bishop Hendricken the following year. Mazzulla made the varsity basketball team as a freshman. He excelled in academics and sports, forming close relationships with teammates.

In Mazzulla’s first high school game, Gomes intentionally kept him out of the starting lineup so he could earn his opportunity. While the idea was smart, it didn’t last long, once Gomes saw how well Mazzulla played.

He started every game for the rest of his high school career.

Mazzulla’s charisma and skill provided a “healthy competition” for teammate DJ Carcieri, who was a year ahead of him at Hendricken. They played together for three years, becoming lifelong friends and appreciating the skills that set them apart in basketball.

“There was always something about Joe — you knew he would do big things,” Carcieri said.

Carcieri recalled how Mazzulla would remember game statistics and nuances from months ago and always had a laser focus during games. Those skills translated seamlessly into coaching, Carcieri said, though he and other close friends never expected to see Mazzulla on the sideline at TD Garden.

“It’s crazy to think that the hometown kid gets to coach the hometown team,” he said. “We used to follow them in high school, and now he gets to coach them.”

Mazzulla has become well-known in the basketball world for sporting a straight face and rarely cracking a smile. Carcieri said Mazzulla’s focus and drive are what make him such a successful coach, even if not everyone understands his style.

But off the court, Carcieri described Mazzulla as a “goofball,” as well as a “tremendous father, wonderful husband, and incredible friend.”

“He laughs a lot more around us than when he’s in press conferences, that’s for sure,” he said.

Carcieri watched Game 5 at TD Garden, alongside Mazzulla’s other close friends and family — quite a change from the games he and his friends have watched from his basement, which even Mazzulla has frequented. Afterward, they celebrated.

Mazzulla’s father, Dan, excelled as a coach at Johnston High School for many years. Before that, he played at Bryant and then professionally in Chile. In April 2020, after a battle with brain cancer, he died at 61.

After the Celtics claimed the title, Dan Mazzulla’s grave was decorated with a tribute to his son’s success.

The hardships Mazzulla has overcome show his strength and dedication to continuing his journey, Gomes said. Mazzulla stayed in close contact after he left Hendricken — first playing for West Virginia University before joining the Celtics as an assistant coach in 2019.

After being named the Celtics’ full-time coach in February 2023, Mazzulla had one call to make ahead of the team’s first practice the next fall.

“Coach, I’d love for you to be there,” Mazzulla told Gomes over the phone. Those eight words were all Gomes needed to hear.

“If you want me there, I’ll be there,” he replied.

Gomes took the early September day off and drove up to the Red Auerbach Center in Brighton for the first practice of the season. He walked up to the mezzanine, passing murals of Larry Bird and other greats, before being greeted by Brad Stevens, the Celtics’ president of basketball operations.

Gomes remembers the first words out of Stevens’s mouth being, “Coach, welcome. What do you think about your boy?”

He answered, “You’ve got a good one, Brad.”

Stevens replied, “I know.”

Gomes drove back to Rhode Island that day in tears, the same kind of joyful tears he shed watching the green confetti explode inside TD Garden Monday night.

Alexa Coultoff can be reached at alexa.coultoff@globe.com. Follow her @alexacoultoff.