Zachary Johnson, an honorary Portland police officer and a popular 2016 graduate of Edward Little High School, died Tuesday after a three-year battle with cancer. He was 18.

Johnson fulfilled his lifelong dream of becoming a police officer in March. Portland City Clerk Katherine Jones swore him in as an honorary officer during a ceremony at police headquarters. Police Chief Michael Sauschuck took off his own badge and handed it to Johnson.

“It was one of those moments the family will never forget,” said an emotional Kevin Haley, Johnson’s uncle and a 21-year veteran of the Portland Police Department. “To see that kid smile from ear to ear, to see him take the oath, it was amazing.”

Johnson, of Auburn, was the son of Aaron and Stephanie Johnson. He also is survived by a brother and sister, Martin and Megan Johnson.

Johnson’s family spoke Friday about the unwavering strength and positivity he showed throughout his fight against Ewing’s sarcoma, a bone cancer.

He was diagnosed in July 2013 at the age of 15. He underwent 10½ months of aggressive chemotherapy and radiation treatment. By April and July of 2014, his scans were clean. A month later, doctors found two tumors in his head. He went through additional treatments before the cancer returned to his pelvis area and then his spine. About eight weeks ago, he became paralyzed from the waist down. Throughout his treatments and setbacks, Johnson maintained an optimistic outlook, his family said.

“He never once said, ‘Why me?’ ” his father said. “He never got down about it. He never got upset. As far as he was concerned, he was going to beat it.”

Johnson had much to live for. He had dreamed of walking in his uncle’s footsteps and becoming a Portland police officer. In March, his wish was granted. Sauschuck gave Johnson his police badge and the St. Christopher medal from around his neck.

“We wanted to step up and officially make him a member of the family,” Sauschuck said. “We really do lean on the core values of our department. When you start talking about leadership, integrity and service, Zach really embodied all of those things. He really brought out the best in people. We certainly saw that when he was here with us.”

Johnson had a ride-along with the department’s Special Reaction Team. Shortly thereafter, he went to the department’s firing range. He also spent time four-wheeling with Maine game wardens, who presented him with a badge and a jacket.

Johnson carried his police badge everywhere, especially to his chemotherapy appointments.

“I’ll tell you, the Portland police family treated Zach like gold,” Haley said. “He was really on top of the world, smiling from ear to ear. He was very blessed to be exposed to all the men and women” in law enforcement.

Johnson attended Auburn schools. On June 4, he graduated from Edward Little, where he was a member of the marching band.

He was escorted to the graduation ceremony by Auburn police. Two classmates pushed Johnson in his wheelchair in the procession line. One of his favorite teachers wheeled him across the stage to receive his diploma. When his name was called, the crowd erupted with cheers and gave Johnson a standing ovation. Many said there wasn’t a dry eye in the stands.

“It was a huge accomplishment,” said Mike Dunn, Johnson’s guidance counselor. “He made our high school community a brighter community because he was in it. Everyone loved Zach.”

John Guy, an ed-tech at Edward Little, said Johnson was the kind of person who thought of others before himself. Guy mentioned the day Johnson called him after the Odyssey of the Mind team Guy coached competed last month in the finals in Iowa.

“He called and asked how the team did,” Guy said. “I was dumbfounded. That’s the kind of young man he was. I will never forget him.”

To honor Johnson and recognize his courage, integrity and service to the community, the Portland Police Department will give him a full policeman’s funeral. Police officers, along with the Maine Warden Service and its honor guards, will pay tribute to Johnson during his wake, which will be held from 4 to 7 p.m. Sunday at Jones, Rich & Hutchins Funeral Home, 199 Woodford St., Portland. A funeral Mass will be celebrated at 10:30 a.m. Monday at St. Pius X Catholic Church on Ocean Avenue in Portland.

Johnson and his family will then be escorted to Gracelawn Memorial Park in Auburn by Portland police, the warden service, Maine State Police, and vehicles from police departments including Auburn, Lewiston and Scarborough.

“It’s quite an honor,” Haley said. “Everyone is willing to do what is needed to show respect for this young man.”