Sinfield, who captained Leeds to their sixth Super League crown last Saturday at Old Trafford, replaces club team-mate Peacock after he announced his international retirement.
“There could have been a number of candidates for the job, when you look at the number of leaders we have in the squad, but Steve McNamara has asked me take up the role and I will do it to the best of my ability.
“When you look at the talent we have in the squad, we are in great shape. We have a big 12 months ahead with the Rugby League World Cup at the end of it and we have to give ourselves every chance of creating history. Steve McNamara is leaving nothing to chance and our preparation continues to be planned in fine detail and I will relish being part of that process.”