Centre Murphy, who has spent this season playing for Barnstaple in National Two South, has been ever-present in the Cornwall squad for the last five years and has helped the Duchy win the county title for the past two seasons.
The former St Boniface School pupil is delighted to have been asked to lead Cornwall.
He replaces back-row forward Kyle Marriott, who has lead the team for the past two seasons.
Murphy said: “It is a great honour to be asked to lead your county and I want to thank the coaches and selectors for showing their faith in my selection.
“Having been a part of this squad for a number of years I know there are a lot of guys who could have been selected.
“I hope that all the supporters will get behind me and the team in the forthcoming campaign and that we can put on the performances that they wish to see.”
Cornwall coach Graham Dawe added: “I am pleased to see Jake selected as the captain. He brings a wealth of experience not only in county rugby but at club level.
“A captain needs to be able to encourage, inspire and lead and Jake has all of those qualities.”
Cornwall will begin their bid for a third successive title away at Surrey on May 6.
They will then face Devon at Ivybridge on May 13 before hosting Herfordshire at Camborne on May 20, in their only home match of the campaign.