The first crash took place on the southbound carriageway shortly before 1.10am this morning, close to the junction for Kirby Sigston.
A black Ford Focus and a lorry collided, causing the Focus to leave the road.
Although the lorry driver did not stop at the time, he has been identified and will be interviewed by police.
The second collision happened close to the junction for Over Silton, also on the southbound carriageway, at approximately 1.20am.
A heavy goods vehicle collided with the man who had been driving the Ford Focus, who was standing on the road.
The man, a 35-year-old from Thirsk, suffered a serious head injury and was taken to James Cook University Hospital in Middlesbrough, where he currently remains.
The A19 was closed until 11am following the collisions.
Traffic Sergeant Richard Parsons, of North Yorkshire Police's Roads Policing Group, said: “I am asking anyone who witnessed either of the collisions to come forward and contact the police straight away.
“I also need to speak to anyone who was travelling on the A19 and may have seen the Ford Focus prior to the first collision or a pedestrian on the road at the time, and urge them to get in touch.”
Anyone who can help with this investigation is asked to contact North Yorkshire Police on 101, select option 2 and ask for Richard Parsons.
Alternatively information can be emailed to firstname.lastname@example.org.
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