Zholia Alemi worked across the UK after claiming to have qualified at the University of Auckland in New Zealand, Manchester Crown Court was told.
Alemi, of Plumbe Street, Burnley, had the power to detain mental health patients against their will.
She had denied 20 offences including forgery but was found guilty by a jury.
She was convicted of 13 counts of fraud, three of obtaining a pecuniary advantage by deception, two of forgery and two of using a false instrument.
At the sentencing hearing Judge Hilary Manley called for an inquiry to be held into how the General Medical Council (GMC) registered her as a doctor, when the documents she submitted in 1995 were "clearly false".
Alemi studied to be a doctor in New Zealand in the early 1990s but did not finish her course, however she managed to work as a consultant clinical psychiatrist.
In 1995 she forged a degree certificate and a letter of verification - with the word verify misspelt, the court heard.
Despite that they were both accepted as evidence by the GMC who registered her as a doctor.
Manchester Crown Court heard she "practised continuously in a very large number of posts literally from one end of the country to another".
The court heard Alemi had earned up to £1.3m in wages from the NHS, which she obtained fraudulently because of her falsified documents.
An investigative journalist uncovered the truth, when in 2018 Alemi was convicted at Carlisle Crown Court of attempting to forge the will and powers of attorney of an 84-year-old widow from Keswick who was one of her patients - so she became main beneficiary of her estate.
She was jailed for five years.
After that Phil Coleman from Cumberland News made phone calls to New Zealand which established the truth, prompting an investigation by Cumbria Police.
The prosecution told the judge that her offences are likely to have led to a loss of confidence in the NHS and "a large number of vulnerable patients have at least been put at risk by being 'treated' by this entirely unqualified defendant".
Following Alemi's 2018 conviction, the GMC apologised for its "inadequate" checks in the 1990s and began an urgent check of about 3,000 foreign doctors working in the UK.
Una Lane, director of registration and revalidation at the GMC, said: "We are very sorry that Zholia Alemi was able to join our medical register in the 1990s, based on fraudulent documentation, and for any risk arising to patients as a result.
"Our processes are far stronger now, with rigorous testing in place to make sure those joining the register are fit to work in the UK.
"It is clear that in this case the steps taken almost three decades ago were inadequate. We are confident that, 27 years on, our systems are robust."