Deacon, 34, enjoyed a decorated playing career as a scrum-half for Bradford and Wigan, before retiring to join the coaching staff of the latter.
His impressive work under Shaun Wane has caught the eye of McNamara - who played with him at Bradford - and he has now been called up to serve his old colleague ahead of this year's World Cup.
The position became vacant after McNamara's former assistant, James Lowes, headed to rugby union to take up a role with Leeds Carnegie.
"Paul is one of the best upcoming coaches in the country with a wealth of experience playing at the top level in the Super League," said McNamara.
"As a player, Paul was a model professional and he is now developing into an excellent coach. I'm sure all the players involved in the England programme will benefit from his vast knowledge.
"He will be able to offer understanding and experience in a number of areas and his insight as a top-class half-back will be invaluable in the work he will be doing with the playmakers in the squad.
"Paul will continue the excellent work developed by James Lowes, the former England RL assistant coach, who we wish well in his new role in rugby union.
"I would like to place on record my thanks to Wigan Warriors and their chairman Ian Lenagan and head coach Shaun Wane for supporting this appointment."
Deacon played for Great Britain 10 times and England four during a career which saw him win numerous titles.
"It is a great honour and a terrific opportunity for me and one I'm really looking forward to being involved with," said Deacon.
"I am looking forward to working with the best English players and passing on my experience. We have some fantastic talent coming through and to be involved at the highest level enabling England to realise its goal of winning a major tournament is something I will relish."