This requires you to win all Festival events, Street races, Showcase events and Showdowns. There are 117 races in total and you need to win them all for the achievement to unlock. The majority of these events will be won as you progress through the game so there shouldn't be too much mopping up/grinding to do.
You can check how many events you have won via the 'Stats' tab in the 'My Profile' menu. You can also check on what events you haven't won by viewing each event via the map. Any event won will have a gold trophy icon next to it.
To make this achievement easier, select the AI difficulty to easy and turn on all assists.