The biggest surprise was the strength of Simpson's victory, who won 45 percent of voters to Cranley's 34 percent, according to unofficial results.
Simpson won without running a single television ad, while Cranley launched an advertising blitz.
"This was sobering for Cranley to have not excelled," said University of Cincinnati political science professor David Niven. "It showed that he is going to have to fight for every single vote. He can't just put his name on the ballot and expect a coronation or a celebration of his four years, which is disappointing for any incumbent."
Another surprise from the night: dismal voter turnout. Just 11 percent of the city's 217,265 registered voters cast a ballot -- far below the expected showing of 15 to 20 percent.
At times, the candidates seemed desperate to boost turnout at empty polling spots.
Simpson used her Facebook page to offer rides to voters, while Richardson promoted a 48-hour blitz of campaigning via Twitter.
But it didn't work.
Fewer people voted in Cincinnati on Tuesday than when the primary was held on the tragic Sept. 11, 2001. And more people -- 28,000 of them -- took to Facebook Tuesday to ‘like' a photo of the Cincinnati Zoo's baby hippo Fiona.
Meanwhile, only 23,399 people voted for mayor.
"It's always hard to get people out for just one race," Niven said.
And many Republicans may have skipped voting Tuesday because there were three Democrats on the ballot.
"We don't know the numbers yet of registered Republicans who came out and voted today," said Republican political strategist Chip Gerhardt of Government Strategies Group. "I would suspect the number was very, very low. I think that would be an area of focus for John (in November). I think that's a natural opportunity for him."
Where do Cranley, Simpson go from here?
The candidate who wins the primary doesn't always become mayor.
In 2005, David Pepper edged out Mark Mallory for the top spot in the primary, but then lost the general election and Mallory became mayor. The same thing happened in 2001 when WLWT anchor Courtis Fuller resoundingly beat incumbent Mayor Charlie Luken in the primary, but then lost in the general election.
"In low-turnout elections, you can have certain people coming out on top who otherwise wouldn't come out on top in a typical election, just because they have a set of core supporters that will always be there for them," Gerhardt said.
Although Cranley may pick up Republican voters in the general election, Richardson voters will likely throw their support to Simpson. Both Richardson and Simpson are considered to be more progressive Democrats than Cranley.
Simpson's strong showing in the primary could also give her a boost with campaign donations. That will help her compete with the million-dollar war chest Cranley is expected to establish by November.
"For Yvette Simpson, this is credibility for her to raise money and to take her message places," Niven said. "It shows that she can actually do this."
Meanwhile Cranley's second-place showing may force him to re-examine his campaign strategy.
"I think he's disappointed. He raised a lot of money and spent an awful lot compared to Yvette Simpson," said Sean Comer, government relations director at Xavier University. "This is something that will make him question his strategy and its effectiveness."
Cranley needs to spend time looking at where he underperformed, Gerhardt said, while Simpson should look at where she performed best and work on an aggressive ground game in those neighborhoods.
But neither candidate should overthink Tuesday's results, given the low turnout, Gerhardt warned.
"There's a lot of value, and I think Yvette's going to go away from this excited about her prospects, and I think John has to go look at the results," Gerhardt said. "Drawing too much of a conclusion from this small number might also be dangerous: Don't get too excited or don't get too depressed."