McCormack, United's leading goalscorer with 18 goals in all competitions this season, will replace Rudy Austin who was appointed as club captain at the start of the season.
McDermott revealed he took the decision to hand McCormack the armband following talks with Austin earlier this week.
"The situation is that I had a good conversation with Rudy Austin - he is a great guy and we had a good discussion."McDermott told the club's website.
"I want Rudy to play his game. He feels his personality isn't conducive to that role.
"You can see his leadership qualities in the way he plays and tracks back. Leeds United means so much to him. I just want to take that pressure away and I want him to play the way he wants with freedom.
"I actually spoke to Ross in April and said 'we need leaders and you are leader.' Whether he saw himself as a leader then I don't know.
"I see him as a talisman and a leader. He has matured an awful lot. Everyone knows what he thinks about Leeds United."
McCormack's first match as skipper will be the lunchtime clash at home to Sky Bet Championship leaders Leicester City on Saturday,
The Scotland international tweeted: "Absolutely delighted to be the captain of Leeds United. Really looking forward to it. A proud day for myself and my family."