Hallissey was the stand-out Brit performer in Sunday's London Marathon - her time of two hours 27 minutes 44 seconds, enough to edge out Jo Pavey for the final spot.
Pavey had taken the gamble to miss Sunday's event believing - and hoping - her time of 2:28.24 from last year's race in London would be enough.
But Hallissey ensured the gamble backfired with her personal best time enough to secure her a London Olympic berth, a spot she plans on fully justifying by going even faster this summer.
"I'm absolutely thrilled, I think it's still sinking in," said Hallissey. "I'm very, very happy. It's a dream come true, it's what you aspire to getting on to an Olympic team.
"I was very happy with my performance at the London Marathon, it went exactly as I was aiming for and hoping for. It was one of those races where everything clicked into place and all went according to plan.
"I've done three marathons so far and I've got quicker over each one so it seems like I've found my distance."
Hallissey, who joins Paula Radcliffe and Mara Yamauchi in qualifying, wasn't the only GB athlete celebrating with Dave Webb also having his 2012 spot rubber stamped by the BOA.
Webb didn't run in London over the weekend but secured an Olympic A-standard by finishing 15th at last summer's World Championships in Daegu.
The 30-year-old used to ply his trade on the track but his South Korean success, plus another top-20 finish at the European Championships in Barcelona in 2010, proved to the selectors his marathon credentials.
"I'm delighted to be selected," said Webb. "I'm going to work extremely hard over the coming months to be in the best possible condition to compete well at the Games."