Preston goalkeeper Andy Lonergan easily saved Adam Johnson's cross low down after eight minutes.
Veliche Shumulikoski fouled Johnson and Gary O'Neil's free kick into the top corner was tipped away after 15 minutes.
Johnson beat Billy Jones and passed to Marvin Emnes and his deflected shot was tipped away for a corner after 22 minutes.
O'Neil's inswinging corner after 24 minutes was punched clear and Middlesbrough goalkeeper Brad Jones saved a Neil Mellor cross after 27 minutes.
Emnes had a shot saved from six yards. Paul Parry had a shot saved after half-an-hour and Ross Wallace had a corner saved after 35 minutes.
Eddie Nolan crossed from the left to Jon Parkin and his effort was saved after 40 minutes.
Parry conceded a free-kick just outside the box and O'Neil scored low into the bottom right, the ball hitting the wall and wrong-footing the goalkeeper after 43 minutes.
Nolan and Neill Collins had crosses saved after 49 minutes and Wallace and Rhys Williams had crosses saved after 55 minutes.
Richard Chaplow replaced Shumulikoski after 57 minutes.
Preston equalised after 61 minutes. Wallace's left-foot cross to the back post found Parry who scored from close range into the bottom right corner of the net.
Joe Bennett was booked for a foul on Parry after 63 minutes and Parry had a cross saved after 66 minutes.
Leroy Lita found Johnson and he made it 2-1 after 67 minutes from close range into the bottom right.
Williams' long-range effort hit the post after 75 minutes and Chris Brown replaced Darren Carter after 77 minutes.
Wallace was booked for dissent after 82 minutes.
Didier Digard was booked for a foul on Wallace after 87 minutes.
Jonathan Franks replaced Emnes and Stephen Elliott replaced Mellor after 90 minutes.
Preston made it 2-2 in the second minute of stoppage time when Wallace found Jones and he headed home from six yards.