Rio Tinto Alcan announced plans to close the site in November.
At 14:00 BST on Thursday, the power was turned off in a ‘low key’ fashion, marking the end of smelting at the site.
John McCabe, the company's regional economic development director, said: "Today is obviously a sad day for all of us concerned, we're bringing 40 years of aluminium smelting to an end at Lynemouth.
"Today we turned off the power to the smelter in a very low key, controlled and above all else, safe fashion.
"Now we will focus our efforts on mitigating the impact of closure on our colleagues and the community at large."
The plant will be decommissioned and cleaned with the aim of bringing new investment and jobs to the site.
The Lynemouth smelter opened in 1972. A total of 515 staff will lose their jobs when it closes, with 323 being made redundant in May.