"The signing of this first NHL contract represents for me another step toward becoming a professional hockey player," MacKinnon said in a statement. "I am excited and thrilled to have been drafted by the Avalanche, and I am already working very hard in order to be ready physically and mentally for training camp."
MacKinnon, 17, led the Halifax Mooseheads to their first Quebec Major Junior Hockey League title and first Memorial Cup championship in 2012-13. He was named MVP of the 2013 Memorial Cup after recording a tournament-high 13 points (seven goals) in four games, including a hat trick and five points in the final game against the Portland Winterhawks.
During the regular season, MacKinnon had 75 points (32 goals) in 44 games for Halifax.
"We are happy to have Nathan under contract and anxious to see him at training camp," Avalanche executive vice president of hockey operations Joe Sakic said in a statement. "His presence will bring additional speed and talent to our team, as well as a lot of excitement to our fans."
Earlier this month, Avalanche coach Patrick Roy said MacKinnon (18 in September) likely would be his third-line center.
"We're going to have to be patient at the start, give him a chance to adapt," Roy said. "I'm not going to compare junior [hockey] to the NHL. Just give him time to feel good."
MacKinnon represented Canada at the 2013 IIHF World Junior Championship in Ufa, Russia. The 6-foot, 182-pound center appeared in six games at the tournament, posting one assist.
MacKinnon is the first player to be selected No. 1 by the Avalanche since the franchise moved to Colorado in 1995. The Avalanche/Quebec Nordiques franchise drafted No. 1 on three previous occasions: 1989 (Mats Sundin), 1990 (Owen Nolan) and 1991 (Eric Lindros).