Freeport native Jon Thompson is the men’s lacrosse coach at Amherst College, which plays for its first national title Sunday in Philadelphia. Clarus Studios

When Jon Thompson left Colby College after two years as head coach of the men’s lacrosse program to accept a similar position at rival Amherst, he did so in the belief that he could build a national champion.

On Sunday, as his ninth season at Amherst comes to a close, he will have that chance.

A native of Freeport who played at North Yarmouth Academy, Thompson has guided Amherst to its first appearance in the NCAA Division III championship game. The seventh-ranked Mammoths (18-3) will face No. 6 Cabrini University (21-2) at Lincoln Financial Field at Philadelphia at 4 p.m. Sunday.

The Cabrini campus in Radnor, Pennsylvania is within 25 miles of the field. The drive from Western Massachusetts, by contrast, takes five hours and covers more than 260 miles.

Not that Thompson is complaining.

On Wednesday afternoon, he brought his team to Gillette Stadium to practice in a National Football League stadium and get accustomed to its size and enormity. For about five minutes, Thompson allowed his assistants to take over.

“I’ve really tried to step back and enjoy this,” said the 38-year-old Thompson. “I looked around and said, ‘I need to soak this in. Because who knows if we’ll ever get back here.'”

Amherst men’s lacrosse coach Jon Thompson confers with a referee during a game against Bowdoin. Clarus Studios

In his nine years at Amherst, Thompson has guided the program to seven NCAA tournaments. The previous four seasons ended in the national quarterfinals. Ranked fourth in the North Region, the Mammoths beat Elizabethtown 21-9 on May 8 and defending champion Wesleyan 16-13 on May 11.

That brought about a quarterfinal date at top-ranked Tufts, the NESCAC champion, but Amherst avenged an April defeat with a 13-11 victory on May 15. In the semifinals on Sunday, Amherst came from behind to beat host Williams, which had won two earlier meetings, by scoring the last five goals of a 12-8 decision.

The decision to leave Maine in 2010 was not an easy one for Thompson or his wife, Susanna, who spent her childhood summers in the state. They were renovating a house in Gardiner. His parents still live in Freeport. Popham Beach, he said, will always be his place to decompress.

In addition, Thompson had turned around the program at Colby, which went 18-13 during his tenure.

“Once you’ve grown up in Maine and get a taste of the lifestyle, it doesn’t ever really leave you,” he said. “I still go back four to five times a year. It’s such a wonderful place to live. The people are so genuine. But this opportunity did not exist in Maine at that time.”

The first time Thompson brought his Amherst team to Waterville, however, he said the Colby community did little to hide bitter feelings about him leaving the program. He said he understands why people were shocked by the decision to leave, because he and his wife experienced some of the same feelings.

“It was not an easy decision,” he said, “but ultimately, it was the right one.”

Thompson has no Maine players on his roster, but he keeps an eye out for potential recruits. He racked up 277 goals and 154 assists during his four-year career at NYA, but won state titles in his other sports, soccer and ice hockey.

At Brown University, he majored in psychology and twice earned all-Ivy League honors as an attackman. He briefly played professionally for the Boston Cannons in Major League Lacrosse while working with at-risk youth at a hospital in Rhode Island.

He decided to pursue further education with a plan of teaching psychology at the collegiate level, and said he fell into coaching first as a volunteer at Brown and then while in graduate school at Springfield College, where he is pursuing a doctorate in sports and exercise psychology.

“No, it was not the thing I wanted to do immediately,” he said. “But when I started doing it, man, it felt rewarding.”

Sunday’s game marks the 168th of his Amherst career. The Mammoths (who assumed that nickname two years ago after dropping the unofficial Lord Jeffs sobriquet) have a 73.1 winning percentage under Thompson, named NESCAC Coach of the Year last spring.

The game will be streamed live on NCAA.com and preceded by the Division II national championship at 1 p.m. between No. 6 Merrimack and No. 1 Limestone.

The Division I semifinals and finals are scheduled at the same site on Saturday and Monday.

“It’s super rewarding to get here,” Thompson said, “but I do feel a real strong sense of the job not being finished. I’m almost not allowing myself to think about what happens if we win.”