Aidan O Shea grabbed the insurance goal for Mayo late on.
All Ireland SFC Round 4B Qualifier
Mayo 3-15 Westmeath 1-14
By Paul Keane at Croke Park
Cillian O'Connor's 1-5 haul was vital at Croke Park as Mayo shook off Westmeath's determined challenge to secure their All-Ireland quarter-final place.
The westerners had to dig so deep, though, as they saw an early 12-point lead whittled down to just three entering injury-time of the second-half.
The end result of a seven-point loss ultimately flattered Mayo who struck 1-1 at the death including an Aidan O'Shea goal to calm their nerves.
Still, Stephen Rochford's side will be delighted to be back in a quarter-final for the sixth consecutive year and they will face Tyrone at 4pm next Saturday afternoon.
It looked as if they would do it the easy way as they roared into a 2-10 to 0-4 lead approaching half-time.
Mayo hit six wides and dropped seven point attempts short in that period, but were well in the lead with goals from Jason Doherty and O'Connor.
Doherty got the first in the 23rd minute after good work by Donal Vaughan and O'Connor capitalised from the penalty spot soon when Westmeath's 'keeper was penalised for over carrying.
Cillian O Connor netted a penalty.
James Dolan stemmed the Mayo tide with a goal for Westmeath and John Heslin added a point to leave Westmeath 2-10 to 1-5 down at half-time.
It was still a huge margin to reel in, but they had momentum on their side and dominated the second-half with Heslin going on to register 10 points for the evening.
He was superb along with Kieran Martin who added two points and the decision to play a more attacking game clearly paid off in the second-half for Westmeath.
Leinster finalists Westmeath lost substitute Denis Corroon to a 64th minute dismissal for two yellow cards, but fought on bravely.
They trailed by just 2-14 to 1-14 entering injury-time after Heslin's 10th point, but O'Connor took centre stage - scoring a point from a free before playing in O'Shea for Mayo's third goal in the 73rd minute.
Scorers for Mayo: C O'Connor 1-5 (1-0 pen, 1f), J Doherty 1-1, E Regan 0-3 (2fs), A O'Shea 1-0, A Moran 0-2, P Durcan 0-1, K McLoughlin 0-1, K Higgins 0-1, D Vaughan 0-1.
Scorers for Westmeath: J Heslin 0-10 (7fs), J Dolan 1-1, K Martin 0-2, F Boyle 0-1.
Mayo: D Clarke; B Harrison, K Keane, K McLoughlin; C Boyle, L Keegan, P Durcan; S O'Shea, D Vaughan; K Higgins, A O'Shea, J Doherty; E Regan, A Moran, C O'Connor.
Subs: B Moran for Boyle (53), A Dillon for Regan (55), C O'Shea for Doherty (59), A Freeman for A Moran (66), C Barrett for Keegan (68), T Parsons for S O'Shea (70).
Westmeath: D Quinn; A Stone, K Daly, J Gonoud; J Dolan, K Maguire, F Boyle; J Heslin, G Egan; J Egan, P Sharry, D Lynch; C McCormack, K Martin, J Connellan.
Subs: D Corroon for Stone (21), D Daly for Lynch (32), S Gallagher for Quinn (h/t), S Corcoran for J Egan (45), D McNicholas for Connellan (53), D Conway for McCormack (66).
Referee: R Hickey (Clare).
