The road begins at Wicksteed Park in the outskirts of Kettering. It then crosses the A14 and goes through Isham and Great Harrowden. After this it goes on to form the Wellingborough western bypass before crossing into the Borough of Milton Keynes (ceremonial Buckinghamshire). From there, it crosses the A428 at a roundabout and cuts through the centre of Olney. South of Olney the road is 40 mph (64 km/h) for a distance before rising to 50 mph (80 km/h) as it passes Emberton, after which it regains the national speed limit (60 mph (97 km/h)). It then meets the A422 at a large roundabout and becomes a dual carriageway. It multiplexes with the A422 to form the Newport Pagnell Bypass. After passing Newport Pagnell the A509 turns left at a roundabout with the A422 continuing ahead. Here it becomes a single carriageway once more and crosses the M1 at Junction 14, where it enters Milton Keynes. Regaining the dual carriageway status it runs for a further 4 miles (6.4 km) past the edge of the centre and the National Hockey Stadium, to link up with the A5 at the National Badminton Centre in Loughton. Through Milton Keynes the road is known as the H5 Portway.
During 2005 there were two fatal crashes in the same 100m stretch of the road as it passes between Emberton and Petsoe End. This area of the road had become notorious as a black spot for traffic accidents, and residents of Emberton had complained to Milton Keynes Borough Council for a number of years for improvements to the road. In July 2005 local residents met in the village and demanded a 40 mph (64 km/h) speed limit and improvements. These were promised by the Council within six months, but were implemented in September 2006.