It starts on the A5 Kilburn High Road, next to the station of that name on the railway line out of Euston. It heads east along Belsize Road parallel to the railway line before reaching a roundabout near South Hampstead station. A short dual-carriageway section follows to reach the Swiss Cottage gyratory on the A41. Westbound traffic on the B509 can go straight whereas eastbound traffic uses most of the gyratory before regaining its number by Swiss Cottage Library.
The road then continues east along Adelaide Road before passing Chalk Farm tube station and ending at traffic lights on the A502.