Youl Mawene rescued a point to keep Preston in with a fighting chance of the play-offs.
But that elusive first away win since January, so vital in the run-in if North End are to claim a top six place, still eluded Alan Irvine's side.
Nicky Maynard headed City in front just before the hour mark after both sides had squandered simpler chances to break the deadlock.
But within 11 minutes Preston were back on level terms when central defender Mawene thundered forward for a left-wing corner by Ross Wallace and stuck out a boot to turn the ball inside the near post.
It was a deserved point, but it could so easily have been all three had Stephen Elliott not clipped a post with a low right-foot shot after dancing between two defenders.
Jon Parkin fluffed North End's best opening of the first 45 minutes, completely missing his shot 12 yards out after Elliott had teed him up.
Dele Adebola somehow put a header over the bar from barely three yards out.
And City top scorer Nicky Maynard squandered another great chance on the strong of half-time, pulling his shot inches wide of the left-hand post when it looked easier to hit the target.
Preston began confidently, but it was City who created the first scoring chance when Ivan Spoule cut in from the right and fired in a shot which Andrew Lonergan had to turn round his near post.
A minute later Chris Sedgwick and Paul McKenna were both guilty of errors to let in Sproule on the other side of the box, but this time his shot flashed across the face of goal with Dele Adebola unable to apply the finishing touch at the far post.
In the 10th minute Robins' keeper Adriano Basso got in a tangle with Lee Johnson on the edge of the six-yard box, but Stephen Elliott's attempt to capitalise was wide of the left-hand post.
Ross Wallace let fly from 25 yards with a left-foot rocket after Elliott had laid the ball back into his path, but keeper Basso was perfectly positioned to beat the shot out.
Adebola was proving a real handful for Sean St Ledger in the air and two headers in the space of a minute set the alarm bells ringing in the North End penalty box.
First he sent an effort wide of the far post after a left-wing cross picked him out on the edge of the six-yard box. But moments later he missed an absolute sitter when a cross from the right found him barely three yards from goal and somehow he nodded over the top.
Just before the half hour mark Jon Parkin, back helping out in defence at a corner, blocked a header from Liam Fontaine in front of goal.
North End had plenty of the ball but were not as creative with it as their hosts.
When Sedgwick released Billy Jones on the overlap on the right the full-back's low cross was comfortably cut out by keeper Basso at the near post.
Sedgwick linked up with Elliott to lay on a half chance for Jones in the box, but his acrobatic overhead kick dropped wide.
Seven minutes before the break Elliott carved out a great chance for Parkin to put Preston in frontwhen he got past his marker on the right and squared inside.
But the big striker tried to pick his spot first time and completely missed the ball!
Then Paul McKenna let fly from fully 30 yards, but as the ball arrowed towards the top left corner, Basso got across to turn it over the top.
On the stroke of half-time City top scorer Maynard squandered a chance to grab the lead, pulling his shot inches wide of the left-hand post from just 10 yards out.
City took the lead 14 minutes into the second half when Adebola got free on the left before chipping into the middle for Maynard to send a header beyond the reach of Lonergan into the bottom right corner.
The goal sparked North End into action and within two minutes Elliott had tricked his way between two defenders to clip the outside of the left-hand post with a right-foot shot.
Parkin unleashed a 25-yarder which beat Basso but also cleared the bar by a matter of inches.
But in the 70th minute Preston were level. Referee Tony Bates gave a corner when it looked as if the ball had come off a North End foot.
Wallace clipped it in to the near post and Mawene came thundering in to stick out a boot and knock the ball just inside the near post.
Loan signing Williamson was thrown straight in at the deep end as Preston faced their latest screen test at Ashton Gate.
Manager Alan Irvine decided to blood the midfielder in the tea-time TV game less than a week after arriving from Watford.
Williamson was brought in to partner returning skipper McKenna in the engine room, with Darren Carter stepping down to the bench.
Simon Whaley also found himself amongst the substitutes, with Sedgwick moving back out to the right-hand side.
The central midfield pair were the only changes to the side which drew 0-0 at Charlton, North End's last game before the two-week international break.
As feared defender Callum Davidson failed to recover in time from his niggling hamstring strain and so Liam Chilvers, out of action for more than a year, kept his place on the bench as the side's only defensive cover.
City boss Gary Johnson kept faith with the 11 who lost 2-1 at Queens Park Rangers a fortnight ago meaning striker Adebola, reported to be on his way out of Ashton Gate at the end of the season, kept his place up front alongside top scorer Maynard.
Bristol City: Basso, Orr, Fontaine, McCombe, McAllister, McIndoe, Johnson, Skuse, Sproule, Adebola, Maynard.
Substitutes: Williams, Trundle, Elliott, John, Henderson.
Preston North End: Lonergan, Jones, Mawene, St Ledger, Nolan, Sedgwick, McKenna, Williamson, Wallace, Parkin, Elliott.
Substitutes: Neal, Carter, Chilvers, Whaley, Mellor.
Referee: Tony Bates (Staffordshire)
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