She took the seat with 91,036 votes with her Labour rival Gareth Derrick on 87,242 votes after both went through to a second round.
There was a turnout of 22.8%, up from 15% in the election of November 2012.
The area's first police and crime commissioner, Conservative Tony Hogg, who was elected in 2012, confirmed last year he would stand for re-election.
Ms Hernandez said: "I'm just delighted that the people of Devon and Cornwall and the Isles of Scilly have put their faith in me to get the best out of Devon & Cornwall Police and keep us all safe."
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More information is available on the Choose my PCC website.
The Conservatives' Alison Hernandez has been elected as Devon and Cornwall's police and crime commissioner.