McCann was born in Lawton, Oklahoma and attended Putnam City High School, where he lettered in football, basketball and track. As a senior, he was rated as the 20th-best player in Oklahoma (Rivals.com), but broke his left leg and was only able to play in 3 games, managing to help his team finish a 10-2 season and reach the state quarterfinals. He turned down offers to play for Oklahoma State, Stanford, Northwestern, Texas Tech, Tulsa, UAB, and Rice, choosing to play for Southern Methodist University instead.
McCann was an All-Freshman Conference USA selection, after having 38 tackles, 4 passes defensed, one interception and appearing in 11 games, starting three of the last four. He was the co-winner of the James Bradley "New Mustang" award for consistent play, along with Emmanuel Sanders and Justin Willis. The next year, he started all 12 games, posting 59 tackles, 7 passes defensed and 4 interceptions, while playing at both corner and safety.
As a junior, he started 11 games, registering 35 tackles, 3 interceptions, one sack and 6 punt returns for 31 yards. As a senior, he had 54 tackles, 3 interceptions and 34 kickoff returns for 824 yards.
After going undrafted in the 2010 NFL Draft, McCann was signed as a free agent by the Dallas Cowboys. He was released at the end of training camp.
On September 5, 2010, McCann was claimed off waivers by the Baltimore Ravens. He was cut after one game (he was inactive).
McCann was re-signed to the Cowboys practice squad on September 16, 2010. He was promoted to the active roster on November 4, 2010, because of injuries to the starting cornerbacks.
In his second NFL game, he recorded his first career interception on November 14, 2010 against the New York Giants, picking off Eli Manning after Hakeem Nicks stopped short on an intended slant route. The interception was returned for a 101-yard touchdown. It broke the Cowboys record of 100-yards once held by Mike Gaechter. McCann’s 101-yard return tied Amos Marsh (101-yard kickoff return) and Ike Thomas (101-yard kickoff return) for the second-longest play in Cowboys history. The longest play belongs to Alexander Wright (102-yard kickoff return). He also became the second rookie in the Super Bowl era to score on a 100-plus yard interception return, the other one was Louis Delmas. For his efforts, he was named NFC Defensive Player of the Week.
The next game against the Detroit Lions on November 21, he grabbed a batted-down punt and returned it for a 97-yard touchdown, which was the second longest punt return in franchise history; only Dennis Morgan’s 98-yard return in 1974 was longer. With this touchdown McCann became the first Rookie in NFL history to score a touchdown of 97+ yards in consecutive weeks. For his efforts his was named NFC Special Teams Player of the Week and the Pepsi NFL Rookie of the Week. He was also the first player in franchise history to win Player of the Week honors in two different categories in the same season, and the first since Troy Aikman, in 1993, to win it in consecutive weeks.
On September 20, 2011, because of injuries to their starting wide receivers, the Cowboys waived him to re-sign Laurent Robinson.
On October 4, 2011, he was re-signed by the Baltimore Ravens. He was released again on November 5, 2011.
On November 17, 2011, he was signed by the Oakland Raiders to replace an injured Chimdi Chekwa. He was one of the last cuts the Raiders made before the start of the 2012 season.
On November 6, 2012, he was signed by the Miami Dolphins. The team didn't re-sign McCann at the end of the season.
On April 4, 2013 he was signed by the Arizona Cardinals. He was released on September 1 and later re-signed on November 19. McCann was re-signed on March 19, 2014. He was cut on August 30.
McCann signed with the Washington Redskins on August 5, 2015. He was waived on August 31.
KeyGP: games played
COMB: combined tackles
TOTAL: total tackles
AST: assisted tackles
FF: forced fumbles
FR: fumble recoveries
FR YDS: fumble return yards
IR YDS: interception return yards
AVG IR: average interception return
LNG: longest interception return
TD: interceptions returned for touchdown
PD: passes defensed