Ken Thompson’s election as Brooklyn’s first black district attorney marked a turning point for the borough. In less than three years, he worked to reform the city’s criminal justice system from within.

Sadly, that work was cut short on Sunday, when Thompson died of cancer at age 50.

He had an impressive record before winning public office. As a federal prosecutor, for instance, he helped prosecute Justin Volpe, an NYPD officer who later pleaded guilty to torturing Abner Louima with a broomstick in the 70th Precinct’s bathroom in 1997.

But it was as district attorney that Thompson came to national prominence. In 2014, he said his office would no longer prosecute arrests for low-level marijuana possession. He overhauled the Conviction Review Unit from a moribund few workers to a model group. He rebooted Begin Again, a program that allowed residents to clear their names from outstanding warrants. On this and other issues, Thompson was an advocate for a fairer criminal justice system that could result in justice, not aggressive prosecution for prosecution’s sake.

Last year, his office tried Peter Liang, an NYPD officer who fatally shot Akai Gurley with a wayward bullet in 2014. Liang’s conviction was welcomed by Gurley’s family and supporters, who later turned on Thompson when he recommended that Liang receive no jail sentence. It was a dramatic study of a district attorney struggling to correct a system but not explode it from within.

The next election for Brooklyn DA is next year. Gov. Andrew Cuomo could appoint a successor or he could call a special election to fill out Thompson’s term, something he usually declines to do.

The office’s chief assistant, Eric Gonzalez, was sworn in as acting DA on Tuesday. Gonzales, a veteran of the Brooklyn office, was promoted by Thompson. Whoever takes over the reins should be an experienced prosecutor with honed skills in the system, someone who would continue Thompson’s reforms. That would be a fitting memorial to a prosecutor whose work was halted too soon.