ROCKLAND — Donna Dennison, the only woman to serve as sheriff in Maine, announced Friday she will not be seeking re-election but instead will retire when her term ends in December.

Dennison said even nearly 12 years after winning a razor-thin victory in a three-way race to become sheriff, she still finds it hard to believe she won.

“I never expected to do as much as I did. The message to young girls is that you can do anything you want,” Dennison said.

Dennison, 65, was born in Waterville and raised in the South Thomaston area. She quit school at 16 years old to get married. She said in a 2010 interview that dropping out was a mistake and if she had any advice for young people it would be to stay in school to increase their opportunities. Dennison later earned her GED.

She grew up on a farm and had wanted to be a veterinarian. She admits she missed a lot by dropping out of school. Her family was poor, with no running water. Water had to be lugged from a well.

Dennison has worked in a variety of jobs over the years. She has worked as a chamber maid, in a restaurant, as a bartender at the former Chuck Wagon in Rockland, and in fish plants. She also worked the midnight shift at the former Crowe Rope in Warren.

Dennison worked at FMC Corporation in the early 1980s until the workers went on strike and the company replaced the striking laborers.

About the same time, then Jail Administrator Albert Hutchinson asked Dennison if she wanted to work as a matron at the old Knox County Jail. She was then offered a job as a dispatcher, which she accepted, but she said she was scared since she had never done anything like that.

She left to help run a small excavation business.

She also raised two daughters during her 13-year marriage.

In 1991, she returned to the county as a dispatcher. A year later, she asked about becoming a part-time deputy.

“Every time I tried something new, I enjoyed it,” Dennison said in the 2010 interview.

A few years later, a sergeant’s position opened and she was promoted and then in 1999 she took a detective’s job.

In 2006, she won in one of the closest elections ever for sheriff, garnering 6,236 votes for a little more than 34 percent of the votes in a three-way contest with independent Todd Butler and Republican Alfred Ockenfels.

She was the first Democrat elected sheriff in Knox County in 70 years and was, and remains, the only woman to serve as county sheriff in the state.

In 2010, she won re-election with 55 percent of the vote. And in 2014, she won by an even greater margin.

Dennison has said that she is most proud of having cleaned up jail operations in her first term. The county had faced multiple lawsuits over illegal strip searches prior to her election.