A first-half double act from Lee Trundle and Michael McIndoe inspired City to secure fourth place.
McIndoe's accurate cross allowed Trundle to slot in from close range in the eighth minute, then Trundle crossed for McIndoe to fire in from 12 yards.
Neil Mellor was twice off target for Preston, while Dele Adebola and Trundle both fired wide for City.
But David Noble found the top corner to seal the win and set-up a play-off against in-form Crystal Palace.
In contrast to the many permutations elsewhere in the Championship, this was not a match of extreme importance.
City were already assured of a play-off place and could not catch Hull in third.
Defeat and a Watford victory could have seen them slip to fifth, but their quick start ensured that would not happen.
606: DEBATE
After Trundle had put City ahead, Preston were twice denied by Brazilian keeper Adriano Basso.
Even after McIndoe made it two, Preston had their chances, Chris Brown volleying over and Neal Trotman heading wide.
Six minutes into the second half, however, Noble made them pay and the game was settled.
The hosts continued to press, however, with Trotman having to block an Adebola header before Marvin Elliott shot wide from the edge of the area.
Preston boss Alan Irvine threw caution to the wind by making a triple substitution shortly before the hour mark bringing Craig Beattie, Darren Carter and Simon Whaley into the action.
City remained the dominant side, though, and Trundle was unfortunate not to collect a second when his effort from outside the area crashed against the crossbar.
Preston keeper Andy Lonergan thwarted substitute Lee Johnson late on, but it was a comfortable day for The Robins, who can now look forward to tackling The Eagles.
Bristol City boss Gary Johnson:
"Lee Trundle worked his socks off and looked a real threat. We played well in defeat at Sheffield United in our previous game, and carried that on here.
"We know what to expect from any Neil Warnock side, and Palace are in very good form. But we are proud of finishing fourth in the Championship after 46 games and will go there next Saturday in confident mood.
"We were going to look forward to the play-offs whoever our opponents were, and the way we played today gives us great heart."
Preston manager Alan Irvine:
"That wasn't the sort of performance that got us out of trouble.
"We have done well recently by looking good when the opposition had possession. Here it was far too easy to create chances against us.
"But the players still deserve a lot of praise, because we were cast adrift at the bottom of the table not so long ago."
Bristol City: Basso, Orr, Carey, McCombe, McAllister, Noble, Elliott, Carle (Johnson 66), McIndoe, Trundle (Sproule 74), Adebola (Showunmi 61). Subs Not Used: Fontaine, Weale.
Goals: Trundle 8, McIndoe 16, Noble 51.
Preston: Lonergan, Hart, Trotman, St. Ledger, Hill, Sedgwick (Carter 57), McKenna, Chaplow, Lewis Neal (Whaley 58), Brown (Beattie 57), Mellor. Subs Not Used: Davidson, Hawley.
Booked: Hill, Chaplow, Sedgwick.
Att: 19,169
Ref: Iain Williamson (Berkshire).
~RS~q~RS~~RS~z~RS~26~RS~)