Smith's late leveller saw Boro climb out of the bottom four on goal difference while the result dents Preston's promotion hopes, despite the visitors extending their unbeaten league run to 11 matches.
Preston took the lead in the 16th minute when Kevin Davies flicked on Paul Gallagher's free-kick from just inside the Stevenage half and defender Tom Clarke slammed the ball in from close range.
The hosts came very close to equalising when Declan Rudd kept out Dean Parrett's close-range header after Roarie Deacon had nodded down Darius Charles' cross.
Salvation arrived for Stevenage two minutes into added time as Luke Freeman's cross was headed down by Charles into the path of Smith, who crashed his first-time effort inside the near post.
The hosts almost won it two minutes later when Rudd had to dive full stretch to keep out Charles' wonderful strike that looked destined for the top corner.