Summerfield has been working as Sturrock's No 2 at several clubs since 2001 and was due to be out of contract with the Pilgrims in the summer.
A club statement read: "The Plymouth Argyle board has been working with manager Paul Sturrock on the shape and structure of the club's football management team, within the framework of a five-year plan designed to fulfil the ambition of bringing Premier League football to Home Park.
"It was agreed that refreshing the approach to coaching would be an important and beneficial first step along this road.
"As a consequence, Kevin Summerfield was advised that his contract as assistant manager would not be renewed when it expired at the end of the current season.
"In light of this, and following discussions between Kevin and the club, it was mutually agreed that Kevin could leave the club with immediate effect to allow him to pursue further career opportunities.
"Paul Sturrock will naturally continue as team manger and lead the work to complete the review of all aspects of the football side of the club, including: coaching; training facilities; the scouting system; medical and physiotherapy; and youth football."
Reports suggest Mariner, who played for Plymouth in the 1970s, will be appointed as Sturrock's new assistant in the near future.
Mariner is currently the No.2 to Steve Nicol at New England Revolution, but the MLS club have given the former England international permission to discuss a coaching position at Home Park.