Bridge won the award for a Tatton Park Flower Show garden based on an iconic English garden brief two line brief.
She says she nearly "chickened out" of entering the competition, which includes a £12,000 grant from the RHS to build the garden, but that the award has now set up her career.
Bridge, 27, from Norfolk, said she took a "wobbly route" into gardening after working with horses but after being given a place on the HBGB course at the royal garden at Sandringham, she designed for the Sandringham flower show, and judges Chris Beardshaw and Alan Mason have her best in show.
She says after finishing HBGB she had the skills to set up her own business gardening.
Bridge joked that the RHS young designer competition "never has a brief that suits me but this time it did, so here we are".
She now has clients she gardens and designs for and also employs a gardener. Her aim is to design at Chelsea. Bridge said Jinny Blom is her design inspiration.
Her ambitions are to become more established: "I'm quite competitive. I push myself. Designing a London show garden - that's what gets me out of bed."