He was replaced by Acting Middlesex County Prosecutor Andrew C. Carey.
The governor's office filed papers on Jan. 2 to nominate Kaplan as a superior court judge in Middlesex County.
Kaplan's leadership as prosecutor drew criticism in New Brunswick in recent years.
Advocates for the family of Barry Deloatch, a city man shot and killed in a struggle with police, claimed the office wasn't doing enough to investigate the circumstances.
When Kaplan left the Prosecutor's Office, the department was dealing with a lawsuit filed by retired investigator Gabriela Nieves, who claimed a hostile work environment developed after she complained about less experienced male colleagues being chosen for promotions over her in 2010.
His office also turned a blind eye to corruption in the Middlesex County Sheriff's Office involving now-jailed former county sheriff Joseph Spicuzzo, according to a report in New Brunswick Today.
Last year, Spicuzzo was sentenced to nine years in prison for soliciting bribes from people seeking jobs or promotions in the county sheriff's office.