Emmett was appointed in 2007. He was then elected to two full terms in 2010 and 2014.
After leading Harris County through the flood bond vote roughly a year after Hurricane Harvey and the plan to revitalize the Astrodome, Emmett's loss came as a surprise.
On Tuesday, Hidalgo will be sworn in.
Hidalgo, who is 27, served as a Spanish-English medical interpreter at the Texas Medical Center and remained in a tight race with Emmett on Election Night.
LINA HIDALGO: What we know about newly-elected Harris County Judge
About a month after his defeat, Emmett announced his plans to be a nontenured professor and senior fellow at the Kinder Institute for Urban Research at Rice University starting Tuesday.