Mayo won an extra-time thriller, beating Kerry by 2-15 to 1-17 to qualify for the NFL Division One final.
Substitute Richie Feeney, late point the westerners a rare Croke Park triumph over their nemesis from the Kingdom.
Mayo led by five points after 12 minutes, with scores from Mortimer, Alan Dillon and Keith Higgins, but once Paul Galvin began to make an impact as an effective link man, Kerry chipped away at the deficit.
Colm Cooper, Anthony Maher and Kieran O'Leary were all on target, before Bryan Sheehan brought the sides level in the 29th minute. But Mortimer brought his tally to six, to send the Connacht men in with a 0-9 to 0-7 interval lead
Kerry opened up a five points lead, James O'Donoghue volleying home a goal in the 53rd minute, but Pat Harte netted a penalty and substitute Killian O'Connor levelled from a free at 1-13 each to send the tie to extra-time.
Kerry eased ahead again, but Feeney broke away on the left in the dying moments to hoist a glorious winner.
Cork will be Mayo's opponents following their 2-17 to 1-12 win over Down in the other semi-final.
Colm O'Neill fired over three points, with Pearse O'Neill and Paul Kerrigan hitting two each as Cork edged a closely contested first half.
Conor Laverty scored three Down points, but it was the Rebels who led by 0-9 to 0-7 at half-time.
O'Neill drilled home a 43rd minute goal as Cork went six clear, but Aidan Carr hit the net from a penalty. Cork secured their place in the decider when midfielder Alan O'Connor plundered his side's second goal 10 minutes from the end, and O'Neill finished with a 1-6 tally.