I believe he was the owner of the factory after WW2.
He aquaintained Aston with the Newport Pagnell Factory, and established the cars in the Lightweight Sport/Luxo-GT car category which prevails untill today.
He oversaw the commany throughout the DB range untill the DB6... the first several cas were heavily involved in competitions, including a LeMans win in 59.
I would say he established a comany with his numerous accomplishments, however I consider the DB4 being his greatest accomplishment. This car not only was one of the highest performing cars of its day, but also established the styling direction for current Astons. The general grille shape, as well as the envelope side scoops you see on your DB9 originated off the DB4. The DB5/6 were heavily based on the DB4/DB4 GT.
The DB7, which came around in the early-mid 90s, the first non David Brown DB, was a ( I think successful) atttempt at reviving the David Brown era, and featured many styling cues of the DB4-6. But here you also have to thank Ian Callum for doing a good job.