The Rocks slipped to their second defeat in five days to crash out of the FA Trophy at the first hurdle.
Matt Groves scored the only goal of the prelimiary round tie for the Southern League visitors after 64 minutes.
The Rocks huffed and puffed but weren’t at their best and now must raise themselves for an important league visit to Ramsgate on Tuesday and next Saturday’s FA Cup visit to Sutton.
Matt Whitehead returned to the starting XI, with Luke Nightingale replacing Terry Dodd, who started on the bench.
The game got off to a bad start for the hosts. Chris Breach gave the ball away in midfield, allowing Tom Knighton to speed down the left wing to cross. But Tim Bond was solid in the box to head it away.
Good work found Dan Beck with space down the left. He found Ben Johnson with a neat pass but the winger was blocked.
The visitors replied when Tom Billings held the ball up well, which led to a Sean Saville cross from the right. Stuart Axten comfortably cleared the danger.
Beck combined with Johnson again on the left. Johnson hit a cross on the bounce, but the ball flew over Nightingale inside the box.
A further chance saw Johnson feeding Whitehead on the overlap, but the left-back’s low cross was booted clear.
On ten minutes, Johnson ended a purposeful passing move to whip in a cross towards Jason Prior. But Gary Colbourne in defence for the visitors got there first before James Crane came in at the back post to head over the bar.
Two minutes later the Rocks were on the attack again. Prior held the ball up before cutting it back to Crane, who smashed a low shot at goal, though it was easily saved by Danny Graves.
Craig Stoner had to pull out a quick save to deny Seaville’s fierce shot towards goal on 18 minutes. Then Knighton beat the defence to the ball just inside the box and his shot on the rise hit the side netting.
Matt Groves’ corner kick flew agonisingly over Stoner and across goal.
Bognor lacked a finishing touch and this was evident when Johnson spun another cross into the area but Graves easily gathered.
Then Nightingale tried to thread in Prior but instead won a corner. Johnson’s first corner from the right was put away for a second on the left. This time he found Axten, who glanced a header towards goal. Breach got the final touch but again Graves saved confidently.
Groves then played in target man Billings who shot narrowly wide of the right post.
Johnson’s sliced shot fell to Prior. He managed to squirm a low shot at goal, though the goalkeeper was equal to it.
Breach lofted a high ball into the box towards Prior. The striker appeared to be impeded but the referee ignored the claims from the home crowd.
Johnson then beat his marker on the left before firing narrowly over the bar from a tight angle. Mangotsfield were a threat on the break. The last chance of the half came when Knighton fired a cross towards Billings, but Bond bravely got there first when he outmuscled the striker.
The first attempt after the restart was when Johnson beat his marker on the right. He crossed to Nightingale who nodded it back to Prior, whose shot was blocked.
Neat play between Groves and Billings saw the latter hit the side netting.
Beck set up Prior from outside the area but the striker blasted way over the bar.
Substitute Terry Dodd had a shot blocked after being played in by Johnson, and then he forced a good save out of Graves.
Seaville believed he was fouled in the area moments later but the referee waved away penalty appeals.
But Mangotsfield took the lead on 64 minutes. Groves got on the end of a square pass on the left to let fly with a beautifully-struck curling shot that beat the outstretched arm of Stoner.
Johnson darted through the visitors’ back-line before scooping a shot narrowly over the bar from a tight angle as the Rocks tried for an equaliser.
The visitors should have gone 2-0 up on 68 minutes. Seaville latched on to a ball on the break in acres of space and picked out Knighton with a cross-field pass. Knighton ran on to shoot low but was denied by the legs of Stoner, who had raced out of his goal.
With less than 20 minutes left, Kane Wills needed treatment after a challenge. This held up play for a few minutes but he did return - only to be shown a yellow card as he got to his feet, presumably for arguing with the referee. Moments later he had to be replaced as he could not run the injury off.
On 76 minutes Crane’s cross found Prior. But his attempt at flicking the ball over marker Colbourne failed. Then Johnson’s cross from the left led to Graves punching clear above Prior.
Billings’ looping header was punched clear by Stoner. Dodd then tried to flick on towards Prior but it was cleared.
Dodd let fly from just outside the box, but it bounced tamely into the goalkeeper’s arms.
Crane got the ball on the right to cross for Axten but he headed over deep into four minutes of stoppage time.
It was a sign the Rocks were never really likely to score against a spirited andf strong-in-defence Mangotsfield side.
ROCKS: Stoner, Crane, Whitehead, Breach (Dodd 56), Axten, Bond, Beck, Wills (Ide 73), Prior, Nightingale, Johnson (Wollers 83). Unused sub: Marzetti.