Britain came into the game fresh from drawing against Germany and beating The Netherlands 3-0, but were 2-0 down to their Belgian opponents in 22 minutes.
Mantell bagged a brace in the final ten minutes though to leave his side unbeaten in the pre-Olympic warm up.
Elliot Van Strydonck tapped in from close range to give Belgium the lead after 15 minutes and Sebastian Dockier extended their advantage seven minutes later, firing home after the ball fell kindly at his feet.
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However, good interplay between Mark Gleghorne and Ashley Jackson in the final quarter allowed Mantell to half the deficit with ten minutes to play.
And the comeback was complete with just seconds to spare as Mantell once again latched onto a Jackson pass to divert it past Vincent Vanasch in the Belgian goal.
Great Britain head coach Bobby Crutchley was left frustrated by his side's performance, but was heartened by the team's fighting spirit.
"I'm disappointed with how we played because we didn't execute our plans and that's a little frustrating," he said.
"At times we overplayed in an effort to get back into the match and that upset our balance.
"It's not quite the display I wanted and we didn't have the control I was looking for but I'm really happy with the character we showed to get something from the game.
"We came here knowing it's an excellent tournament and it's against three of the best teams in the world so in that respect it's been a superb competition for us.
"We've been reminded of what we need to work on and we're in pretty good shape for the challenges ahead.
"Rio is a long and very difficult tournament."
Germany beat the Netherlands 3-2 in the tournament's final fixture to leave Great Britain finishing in second, two points behind the hosts.
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