McCann's touchdown at 9:16 of the third erased a 12-7 deficit and sparked the Cowboys to their second straight win with Coach Jason Garrett at the helm.
Jon Kitna gained 147 yards, passed for three touchdowns and ran in another one for Dallas (3-7), which exploded for 28 points in the second half.
Miles Austin caught a pair of second-half touchdowns and Felix Jones gained 51 rushing yards for the victors.
Shaun Hill picked up 289 yards and fired two touchdowns for the Lions (2-8), who extended their NFL record for road losses to 26 straight games.
Ray Liotta sues skin care company over use of likeness
Boston schools pull out free condoms over wrapping complaints