The 188-game veteran missed the event because his partner Anna went into labour with the couple's first child this afternoon.
Fellow long-time Warrior Sam Rapira, who captained the side against Canberra earlier in the season, accepted the award in his team-mate's absence.
Mannering, 27, who won the award in 2008 and 2011, was one of four finalists along with other long-term NRL performers Nathan Friend, Thomas Leuluai and Manu Vatuvei.
"The quality of Simon's work and his sheer capacity for work is exceptional," said Warriors head coach Matthew Elliott.
"I've long admired what a fantastic player he is from a distance but that respect has gone to another level after having the privilege of working with him for the first time."
Mannering now ranks third on the club's list of all-time appearances behind only Stacey Jones (261) and Logan Swann (195).
He finished the year sitting on 88 appearances as captain since taking on the role at the start of the 2010 season to leave him just three games short of drawing level with legend Steve Price for the club record of most games in charge (91).
Vatuvei was named Clubman of the Year and Ngani Laumape was Rookie of the Year.
