Priestley served for 10 years as secretary general of the European Parliament and was an early President of the Young European Federalists in the mid -seventies.
Priestley held the highest office in the secretariat of the Parliament from 1997-2007, a time of growth as the EU expanded from 15 to 27 states and the Parliament gained in strength and influence.
After a spell in student politics - he was president of Oxford University's debating society, the Union - Priestley seemed destined for a career in the UK Parliament and was three times a British Labour party candidate in Plymouth, his home town in the south east of England.
Priestley led the student campaign in the 1975 UK referendum for its EU membership and, after then launching a career in Brussels, he worked for a period in the secretariat of the budgets committee.
He then became Secretary-General of the Party of European Socialists (1989-94), then head of the private office of Klaus Hänsch, when the German centre-left MEP was president of the Parliament from 1994 to 1997.
Former ALDE group MEP Andrew Duff led tributes to the much-liked former civil servant.
On Monday, Duff, now a visiting fellow at the European Policy Centre in Brussels, told this website, "I knew Julian since we were both involved in the 1971-72 student campaign to join the European Community - he at Oxford, I at Cambridge.
"His record of service to the European Parliament was remarkable, and he will be remembered as a key figure in that remarkable class of British Europeans who, in spite of the odds, made a major contribution to the political integration of Europe."
Paul Adamson, founder and editor of the Brussels-Based E!Sharp magazine said Priestley's death was a huge loss.
"I last saw Julian six months ago when he agreed to do a podcast with me for E!Sharp. Although he had been seriously ill for some time he had fire in his belly as he condemned with the utmost contempt the dishonesty of the political class for engineering the calamity that is Brexit."
He was, suggests Adamson, "that ultimate paradoxical figure: a dedicated left-leaning pro-European and the quintessential civil servant who nonetheless preserved his acute political reflexes until the very end."
Further tribute came from German Socialist MEP Jo Leinen, who said, "Julian Priestley was a dedicated pro-European. From his student days until his last moment he believed in a united Europe and worked for it.”
Leinen added, "We lose a very passionate and competent comrade, we will keep Julian in good memory."
Fellow S&D group MEP Richard Corbett said he had known Priestley for more than 40 years and had remained close friends ever since.
"I first met Julian when he gave a speech in Trafalgar Square in the 1975 referendum campaign. After speeches by Edward Heath (Conservatives), Roy Jenkins (Labour), and Jeremy Thorpe (Liberal), he spoke for the Youth for Europe campaign - and gave by far the best speech."
The British MEP added, "Although he had opportunities to enter British politics - and he would have reached the highest levels - he chose to dedicate his career to Europe, and in particular building a democratic Europe with a central position for the elected Parliament.
"He will be remembered as one of the Brits who helped build the EU as we now know it, sadly just as we are likely to leave."
Socialist group leader in the European Parliament Gianni Pittella told this website, "Julian Priestley will be deeply missed. He was an outstanding European. An engaged and brilliant parliamentary official in the many different functions he carried out including as the parliament's Secretary General but also as Secretary General of our Group.”
He added, “His intellect and brightness as well as his never ending optimism will be missed; he inspired so many of us in the House and beyond."
"He was very clear in his conviction - that the European Union is the best achievement to ensure peace and prosperity in Europe. A project that of course needs reform - and being member of our group and the British Labour Party he always fought for a more social Europe - but that the EU is a project that needs to be maintained. He played a key role also as Head of the electoral campaign team for Martin Schulz as EU Commission President, and did not keep silent around the Brexit referendum either but continued to fight his pro-European case."
"We will miss him and will make sure that his optimism and European inspiration will live on".