After serving as Kent County’s prosecutor for 30 years, William A. Forsyth on Friday heads into retirement.

Forsyth, 67, came to Grand Rapids 40 years ago and served as an assistant prosecutor for a decade before taking the top spot. During that time, he’s personally tried more than 150 cases, including some of Kent County’s most notorious.

The first person he tried as prosecutor was the man who gunned down Grand Rapids Police officer Joe Taylor in November, 1986. The final case he tried was the man who fatally shot Grand Rapids officer Robert Kozminski in July, 2007. He won convictions in both cases.

“It wasn’t factually that either one was that difficult; I’ve tried much more difficult cases factually,’’ he said. “But emotionally, those would have to be the two that were toughest.’’

Forsyth, who grew up in Standish and got his start as an assistant prosecutor in Lapeer County, plans to remain in the area after he hands the keys over to Chris Becker, his successor, on Friday, Dec. 30.

The job, he said, “I’ve loved from the very beginning.’’

He counts the conviction of Albert Lee for the 1979 murder of 11-year-old school crossing guard Linda VanderVeen among his most satisfying wins. He spent several weeks preparing for the trial and lost a dozen or so pounds in the process.

“That one I will say, from the difficulty of the facts and the pressure, was probably the most difficult case that I’ve ever tried,’’ Forsyth said.

A Walker teen who butchered his mother and two sisters and a Sparta teen who killed a young man and then recorded himself talking to the severed head are among his most gruesome cases, Forsyth said. Both were convicted and sentenced to life in prison.

Jon D. Siesling, then 17, fatally stabbed his mother and two sisters inside their home on Walker Avenue NW in Jan. 2003.

Federico Cruz, then 16, met and killed a Sparta teen along railroad tracks near the Sparta Airport in April, 1996. Cruz made a videotape of himself talking to the victim’s severed head. “Basically narrate yourself mutilating the skull? I mean, it’s about as bizarre as you can get,’’ Forsyth said.

When he started, the Kent County Prosecutor’s Office had 14 assistant prosecutors. Today, there are nearly three dozen. His office reviews about 10,000 cases a year.

“There are a lot of decisions in this job that are not easy; but you just simply have to make the call and move on,’’ he said.

As a release, Forsyth likes to run. He was in his 50s when he finished the Boston Marathon in under 3½ hours. He and his daughter climbed Mount Rainier about eight years ago. It was not uncommon to see him running through the streets of Grand Rapids during his lunch hour.

Forsyth plans to return as a special prosecutor next year to handle the cases of 13 teen killers he says should never get out of prison, including Siesling and Cruz. Those cases are on hold until the Michigan Supreme Court decides whether they should be re-sentenced by a judge or a jury.