Coach Brad Arthur also appointed an eight-man leadership group for the first time in the club's history.
The group includes Scott and Mannah, Corey Norman, Frank Pritchard, Issac De Gois, Tepai Moeroa, Manu Ma'u and Michael Jennings and will play a pivotal role in maintaining high standards and a strong culture on and off the field.
"Beau and Tim are admired and respected by their teammates and coaches alike and I look forward to working closely with them over the course of the season," Arthur said.
"In 2017, we want to build the leadership capability amongst our team so I am pleased that we have appointed a leadership group with an energetic mix of respected senior players and emerging leaders, all with their part to play in our club's development."
Mannah said he was honoured to again be named as the club's captain.
"I am looking forward to the challenge that Beau, myself and the leadership group have in maintaining and driving the elite standards of this club," he said.
"We have a responsibility to set a healthy example to the playing group on what we stand for as a team and as a club."
Mannah was named co-captain alongside Kieran Foran last season before the former Manly playmaker left halfway through the year. Mannah took over the captaincy alongside Jarryd Hayne and Reni Maitua in 2013 after Nathan Hindmarsh's retirement.