As a 19-year-old at the 1960 Games in Rome, Lonsbrough won gold in the 200m breaststroke in a world record time.
The Yorkshirewoman, who combined her job as a clerk in the borough treasurer's office at Huddersfield Council with training three times a day in the local baths, had shot to prominence with two golds medals at the Commonwealth Games in 1958.
Lonsbrough qualified for the final in Rome in the second fastest time behind world record holder Wiltrud Urselmann.
Urselmann led early on before Lonsbrough came through on the third length and then held off a late rally from her German rival to take a thrilling victory by half a body length in a new benchmark of two minutes 49.5 seconds.
She added the European title in Leipzig in 1962 and claimed two golds and a silver at that same year's Commonwealth Games in Perth.
Lonsbrough later married world pursuit champion cyclist Hugh Porter, the couple having met when both were part of the Great Britain team for the 1964 Olympics in Tokyo and taken their vows the following year.
She was a trailblazer for women in British sport, becoming the first to be named Sports Personality of the Year in 1962 and carrying the Union Jack at the opening ceremony in 1964 - another first.
Lonsbrough was awarded the MBE in 1963 for services to swimming and went on to enjoy a successful career as a journalist after retiring from the pool.
She remained the only British woman to win an Olympic swimming title for 48 years, until Rebecca Adlington matched her feat - and then added a second gold for good measure - in 2008.