Dr Michael Irwin, 79, paid £1,500 towards the cost of 58-year-old Raymond Cutkelvin's death at the Dignitas clinic in Switzerland.
He vowed to highlight the "hypocritical British system" surrounding euthanasia as he was arrested and questioned by police.
Director of Public Prosecutions Keir Starmer said there is sufficient evidence to prosecute him but it would not be in the public interest.
Mr Cutkelvin's partner of 28 years, Alan Cutkelvin Rees, was also told he will not face prosecution.
Mr Cutkelvin, of Hackney, east London, was diagnosed with an inoperable tumour of the pancreas in 2006 and died the following year at the clinic.
Mr Starmer said no criminal complaint was made about Mr Cutkelvin's death and police only began an inquiry after a newspaper article was published.
He said Mr Rees collected information, used a joint account to pay more than £3,000 towards the costs and accompanied Mr Cutkelvin to Switzerland.
But he said the dead man was "strong-minded" and made an "informed decision" to commit suicide "without any pressure" from Mr Rees or anyone else.
He said: "Mr Rees acted throughout as a supportive and loving partner and was wholly motivated by compassion."
Speaking about Dr Irwin, he said the circumstances were more complex, but that he too should not be prosecuted.
He said Dr Irwin cooperated with police and but already has a caution for assisting suicide.
Mr Starmer said at his age it is unlikely a court would impose anything other than a "nominal penalty".
He said: "As I have stated previously, applying the policy is not simply a matter of adding the factors for and against prosecution - they must be considered in the unique circumstances of each case and nothing in this decision should be taken as an indication that particular acts will not be investigated in the future or that they would not form the basis for a charge on other facts."
Dr Irwin was struck off the medical register in 2005 by the General Medical Council (GMC).
He had travelled to the Isle of Man with the intention of giving his friend, Patrick Kneen, up to 60 Temazepam sleeping pills to help him die.
But Mr Kneen, who was in his late 70s and had prostate cancer, was too ill to take the class C drug and died a few days later while in a coma.
The GMC struck Dr Irwin off the medical register, saying his actions had been "unprofessional", "inappropriate" and "irresponsible".
Dr Irwin, of Cranleigh, Surrey, stood down as chairman of the then Voluntary Euthanasia Society, now renamed Dignity in Dying, after receiving a police caution.
In March, the son of conductor Sir Edward Downes was told he would not be charged with assisting his suicide after booking their hotel room and accompanying them.
Sir Edward died with his wife, Lady Joan Downes, at the Dignitas clinic in Switzerland on July 10 last year.
Mr Starmer said there was sufficient evidence to prosecute Caractacus Downes but it was not in the public interest to do so.
Mr Starmer published new guidelines in February that outlined that motive should be at the centre of any decision over assisted suicide.
They stated that anyone acting with compassion to help end the life of someone who has decided they cannot go on is unlikely to face criminal charges.
The document was published after a Law Lords ruling in favour of Debbie Purdy, who has multiple sclerosis.
She wanted to know whether her husband would be prosecuted for helping her to end her life.
Assisted suicide remains a criminal offence in England and Wales, punishable by up to 14 years in prison.
Sarah Wootton, of Dignity in Dying, said: "Dignity in Dying believe that people should not be forced to take the law into their own hands to have what they consider to be a dignified death.
"Furthermore, terminally ill adults suffering at the end of life should not have to travel abroad to die.
"The decision not to prosecute either Mr Rees or Dr Irwin demonstrates that following the Director of Public Prosecutions' guidelines on assisting a suicide, compassionate assistance to die is unlikely to result in a prosecution.
"However, Parliament cannot continue to bury its heads in the sand and pretend that people are not taking drastic and sometimes dangerous decisions.
"Not only are Britons travelling abroad to die, but here in the UK terminally ill patients, their loved ones and their doctors are taking matters into their own hands."
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