The Valkyries had to soak up a lot of early pressure before falling behind to a penalty.
They hit back though with what proved to be the only try of a close-fought encounter.
Bronwyn Jacobs made a superb break and Laura Ineson was on hand to finish off the move.
A penalty for offside, however, saw the Valkyries trailing, to their Bristol-based opponents, 6-5 at the break.
Injuries forced a reshuffle but despite this as the game entered the closing stages it was the Valkyries doing all the pressing.
Natasha Richardson crashed over the St Mary's line but was adjudged to have dropped the ball.
With time running out the Valkyries were awarded a penalty 20 metres out and Chloe Stopard held her nerve to slot the ball between the posts.
Stopard sealed the win with another kick at the death.
Senior coach Mark Witcher felt it was a just reward for all the hard work put in by the players.
He said: ‘It was a brilliant game of rugby between two unbeaten teams. Either team could have won it and it was a fantastic advert for Women’s rugby.
‘Our players were dead on their feet at the end. Some of them had been involved playing for the Navy and RAF on the Friday.
‘They have put in the effort and hard work week after week.
‘As coaches we knew they were capable of achieving it.
‘It is the 20th anniversary of the Valkyries and this puts the icing on the cake.
‘We know we now have a lot of work to do moving up to the next level.
‘At the moment we are just going to enjoy the moment.’