Based on research he had published in 2016, Mr Mansillo found support for the monarchy in Australia began to wane in the 1960s and crashed to a low about the time of the republic referendum in 1999.
However, since then, there's been a slow but steady improvement aided in part by events including Prince William and Kate Middleton's wedding in 2011 and the births of their three children.
"Events such as royal weddings contribute to improvements as people get to witness the grandeur, the splendour, the pomp and ceremony and this self legitimises the institution," Mr Mansillo told AAP.
"After Kate and William's wedding I found that there was a pretty big bump in the number of people who saw that royalty was important, a seven to eight per cent increase in how many people who thought that what these people do for Australia is important."
Mr Mansillo's research, published in the Australian Journal of Political Science, found support in Australia for the monarchy hit its lowest point about the turn of the century.
During the 1990s, the royals were rocked by scandals leading up to the 1996 divorces of Prince Charles and Princess Diana, and Prince Andrew and Sarah Ferguson.
It was also the decade that debate about Australia becoming a republic ramped up before the 1999 referendum.
At the same time, there was a sharp dip in support among Australians for retaining the monarchy and those who believed the Queen was important, Mr Mansillo's research found.
However, that fall in support bottomed out about 2001 and 2002 and has risen steadily ever since.
Mr Mansillo attributes that to many young Australians, particularly generation Y, having no memory of the royal scandals of the 1990s.
"And because we don't have royal scandals (the Australian Republican Movement's national chair) Peter FitzSimons can't get up and complain about it with a really, really big megaphone," he said.
"So there's fewer bad media images and stories about the royals coming out from London and more good stories which make it very difficult to campaign against."
The most recent Newspoll on support for an Australian republic, published in April, found support for the monarchy was at 41 per cent - its highest level in 18 years.
Fifty per cent said they wanted a republic, with nine per cent uncommitted.