He worked his last day on Friday after the trade body decided to move from near his home in Cambridgeshire to the west Midlands.
"This has been a difficult decision for me to take, as I have felt very much at home in the industry," said Marshall.
"Unfortunately, I have done long distance commuting before, and my judgement is that it would not permit effective working at a time when the industry is facing growing challenges. I have therefore reluctantly exercised a break clause in my contract.
"It would be wonderful to remain within the recycling sector, as the future looks very bright indeed."
Previous director general Rick Wilcox has temporarily reassumed the position until a permanent successor is found.
Marshall's finest hour came in July last year when he negotiated an agreement with the Environment Agency (EA) that ended six days of switched-off shredders.
New hazardous waste laws had left vehicle recyclers unable to find any landfill sites to take their shredder waste until an interim compromise was found.
Sims Metal managing director Tom Bird said then: "Neil Marshall has done an excellent job on behalf of the metals industry. It has been particularly unpleasant but a solution has been found."
Fittingly, a final solution to the issue of shredder residue was announced by Marshall this month.
More than six months of work paid off when the EA said fully depolluted vehicles would count as non-hazardous.
Marshall also worked tirelessly to secure a fair deal for vehicle recyclers from the End-of-Life Vehicles Regulations.
This issue was also finalised in the past fortnight, with the final regulations becoming law and making manufacturers responsible for recycling their own cars.
Marshall said: "Unfortunately it still looks like the recyclers will be bearing the majority of costs. It's just appalling."
BMRA President George Bond said this week: "I am naturally disappointed that Neil has resigned, as he has been an extremely effective focal point for the industry.
"However, I respect his need to meet family commitments in the Cambridge area and we will miss his very evident skills.
"Neil will be a very tough act to follow."