"It was only when I retired that I went back to the university to say I wanted to finish what I started."
Dr Lester, who sold his pharmacy business and became chair of an NHS trust in Harrogate in the 1970s, said it was his time working in the NHS that inspired his doctorate subject.
"When I started I wanted to do a PhD on the effects of drugs on the brain but the tutors said to me, 'a lot has changed since you first started your studies'.
"One of the professors had an idea and encouraged me to look into the work of non-executive directors on NHS boards."
Dr Lester, who attended last week's graduation at the Royal Festival Hall with his five grandchildren, said he was worried that now his studying was over he would have nothing to do to keep busy.
He said: "To be honest I'm bored now. When you retire you have to try to keep busy and I don't play golf or anything like that.
"One of the great things about doing it was constantly reading and writing and talking to people.
"It is true what people say - if you don't use it you lose it and I can tell you there is nothing wrong with my brain."
He said he took more than six years to complete the PhD, adding that his motivation "was always my grandchildren. I wanted them to see that education never stops.
They thought it was wonderful to see me up there and I hope it encourages them to always keep learning."
The Harrogate Hebrew Congregation member said: "One of the things that emerged from my research was that there are very few health care professionals in non-executive boards on NHS trusts.
"I'm not expecting anything to change based on my research, but if people want to have a look and something comes from it, why not."
