The Australian surprised many by qualifying fastest on Saturday but backed it up with a time of 4:35.580 in the finals that was over a second faster than fellow Aussie Sam Hill in second and Britain's Danny Hart in third.
Aaron Gwin, the overall race leader, and Gee Atherton rounded off the podium while Greg Minnaar just missed out at his 100th World Cup race.
The result makes the race for the World Cup title very interesting, in what has already been one of the most competitive seasons ever, with three different winners so far.
This is something I’ve wanted to do ever since I was a kid
But, although the 20-year-old Brosnon insists he his not thinking beyond the next race, he is confident he has proved a few people wrong in Fort William.
“This is something I’ve wanted to do ever since I was a kid and started riding downhill,” he said. “To do it has made me smile and given me confidence for the next world cups.
“After qualifying first everyone knew I could do it but it’s another thing to go ahead and actually do it.
“This will build my confidence and hopefully it will open other peoples' eyes. We’ll take it race by race from here but it’s going to be hard because there’s a lot of guys who will go fast."
The British crowd of around 9,000 people had plenty to cheer about in Fort William with plenty of home grown talent performing well.
One of the first to make the hot seat was Sam Dale, who’s powerful bottom section was too much for countless riders, until Minnaar edged him off the top by half a second.
However, five riders later he was ousted by Canadian Stevie Smith, making his first appearance at a World Cup this year after recovering from an ankle injury.
The defending champion was then pushed into second by Atherton before Gwin secure his lead at the top of the overall by crossing the line 0.6 seconds faster than his British rival.
After an unlucky flat tire for Sam Blenkinsop, Hill took the hot seat before fan-favourite and 2011 World Champion Hart moved into third leaving Brosnan to take his first title.
In the women's elite race, Emmeline Ragot won her first World Cup race of the season to blow the overall standings wide open after Rachel Atherton and Manon Carpenter both suffered flat tires.
The French rider, who finished second behind Atherton in the 2013 World Cup overall, beat Myriam Nicole to the top of the podium with Tracey Hannah coming in third.
But the shock of the day was three mechanicals to British riders as Tahnee Seagrave, Atherton, and fastest qualifier Carpenter, all sustained flats while looking strong.
Despite her bad luck Carpenter moved into the overall lead because Atherton ended up being disqualified for allowing a spectator to help correct her mechanical.
Australian Hannah set the early pace before Ragot smashed her time by 13.9 seconds to take the hot seat.
Penultimate rider down Nicole struggled to get close with Ragot eventually proving a to be a clear winner while also ensuring there will be an exciting chase for the overall title.
“It always feels good when you get a win and especially here,” she said. “I had a good run and really pushed myself.
“The overall should be good now. I’m 28 and I want to get a title before I stop.
“Everyone got stronger over the winter and it’s good to see everyone fighting and knowing that four or five girls can win.”