In his first start after moving on loan from Southampton, his second-half strike pushed City's promotion hopes further towards the horizon.
Worse still they picked up four bookings and for three of them, player-manager Brian Tinnion, midfielder Tony Dinning and skipper Jamie Smith, it means they will miss the next match at Colchester.
Walsall always looked like winning and a deep left-wing cross from Darren Wrack gave them the break they needed as Surman out-jumped his marker Bradley Orr in the 75th minute.
Walsall's player-manager Paul Merson opted to leave himself out of the side and kept his young leading scorer Matt Fryatt on the bench until 22 minutes from time to rest him after his under-19 international earlier in the week.
With their poor away form it looked as if Walsall were on a damage limitation exercise with Jorge Leitao as the only striker.
It was a deceptive message as they quickly took over the running. Leitao had the ball in the net inside four minutes but he had been beaten by the offside flag.
Fed by Surman he was soon in with another attempt, this time rolling a shot a foot wide of the far post with City's defence nowhere.
With Wrack rifling a shot just wide of the other post, Bristol City were close to serious trouble and the only sign of composure they had was in Louis Carey - the defender just re-signed from Coventry.
It was an uncomfortable night for Bristol City and this was highlighted when Surman beat Tinnion in midfield to set up another dangerous raid.
City were reduced to snatching at half chances, a weak header from Steve Brooker which reached the keeper on the bounce was just about the best of them in the opening period.
After his goal for the England under-21 side, Leroy Lita had a very quiet night against some tight marking.
Tinnion's booking came after he caught keeper Scott Murray on a long sliding challenge.
That might have been marginal but the other bookings, which also included one for centre-back Craig Ireland, were not.
Torrential rain never made it easy for either side but Walsall dominated the second half as Bristol City used up all their substitutes inside the hour.
In a desperate final rally, Ireland went up to join the attack and struck a post with a left-foot shot but City were never confident enough to rescue the game.