As a prosecutor for more than three decades, Alexandria Commonwealth’s Attorney S. Randolph Sengel has handled hundreds of jury trials, from shoplifting cases to murders.

A few cases, however, stand out. In his office, Sengel keeps a picture of 3-year-old Katelyn Frazier, who died of abuse in 2000 in her mother’s apartment, even though she was in city custody.

“She was the same age as my granddaughter at the time,” Sengel, 63, said Tuesday.

In an interview, Sengel reflected on his legal career, which will conclude at the end of 2013 after his decision not to seek another term. He has spent 33 years in the office, 15 as commonwealth’s attorney.

In Katelyn’s case, Sengel got a guilty plea from the mother and won a conviction in the case of the woman’s boyfriend.

“There are no crimes that, at the end of the day, make sense,” Sengel said. “But for those crimes, you really have to grit your teeth.”

Another case Sengel recalled was from January, when Brian Hernandez-Chavez, 8, was killed by a drunk driver on a Sunday afternoon as he and his mother walked to a playground. The driver, Victor Aldana, was convicted of manslaughter while driving under the influence and other charges.

Sengel and the 12 lawyers in his office have handled their share of high-profile matters, including the case of former New York congressman Vito Fossella. Fossella’s drunken driving arrest in Alexandria led to revelations that he had fathered a child in an extramarital affair, which prompted his political downfall. Fossella eventually pleaded guilty.

Sengel said his philosophy of running the office has been to develop the lawyers, focus on the cases and try to keep politics out of it.

Arlington Commonwealth’s Attorney Theo Stamos, who interacts with Sengel frequently, called him a “great student of the law.”

“He’s not a large personality, but he’s really a speak-softly-and-carry-a-big-stick kind of guy,” Stamos said. “He can cut to the chase and make a cogent argument in fewer words than any lawyer I know.”

When Sengel was deciding in 1997 whether to run for the job, he hesitated because he wasn’t sure he’d enjoy politics. But he ran — and won — that year, and he hasn’t had an opponent in an election since.

“It’s either because nobody wants the job or because we’re doing a great job,” he said.

In the interview, Sengel said the years he’s served as commonwealth’s attorney have been as he expected: He was fond of the cases — even the difficult ones — but not so much the ceremonial aspects of the job.

“One year they asked me to be in a parade, so I brought my grandchildren and put them in the car with me,” said Sengel, who has a quick wit and a monotone delivery. “They immediately fell asleep.”

Sengel gave a nod to Assistant Commonwealth’s Attorney Bryan Porter, whom he said he would support as a candidate for the top job.

Sengel grew up in Alexandria, graduating in 1967 from T.C. Williams High School, the same school attended by Alexandria Mayor Bill Euille and Alexandria Police Chief Earl L. Cook.

After graduating from Williams College, he attended divinity school at Yale University for a year, following in his father’s footsteps. He then left and earned a law degree from the University of Virginia in 1976.

He went into private practice for a few years, handling criminal defense cases, domestic violence matters and real-estate transactions. In 1979, he joined the Alexandria Office of the Commonwealth’s Attorney, taking the General District Court docket and slowly working his way up. In 1985, he became deputy commonwealth’s attorney, and campaigned for the top job in 1997 when John E. Kloch left to become a judge.

In his retirement, Sengel said, he plans to continue traveling and fishing in remote parts of Alaska where there is no cellphone service. He and his wife, Noel Sengel, a retired lawyer, plan to stay in Alexandria.

“We recently renovated our kitchen,” he said.