Scott Mann, Conservative MP for north Cornwall, has confirmed he will resign at a full council meeting on February 16.
The Wadebridge West councillor said doing both jobs was only an "interim arrangement".
But he has faced criticism over his attendance record at council meetings.
Between July and January he only attended three meetings at County Hall - one of the worst records.
"I knew that doing both roles was only an interim arrangement once I had tied up my commitments I made to the town's residents," Mr Mann said.
"I made it clear to residents that I would endeavour to find a solution to their parking problems, and I planned to use a highways budget that was subsequently withdrawn after the election.
"Wadebridge is now part of a countywide parking review and it seems fit to step aside now."
Mr Mann has represented the ward since 2009 but came under fire after being elected MP for north Cornwall last year.
Adrian Jones, chairman of the north Cornwall Labour Party, previously told the BBC "it was obvious an MP cannot do both roles".
It is not yet known when a by-election will take place.
An MP is to step down from his position as a councillor in Cornwall following calls from local politicians for him to leave.