With all 173 precincts and absentee ballots counted, Grossmann defeated former State Rep. Ron Maag, 12,017 to 9,343, according to unofficial election-night results.
There is no Democratic candidate, so the race should decide who fills the seat, one of three on the commission, through 2022.
The results indicate Grossmann, 61, won with about 56 percent of the vote, the margin by which he won four years ago in a primary race with incumbent Tom Ariss, who had also been the county sheriff.
“When you beat somebody like that, it’s a huge victory,” Grossmann said afterward.
Maag, a four-term state representative, could not be reached on election night.
Maag, 72, was term-limited in the Ohio House of Representatives, but hoped to extend his political career in Columbus through appointment to replace Shannon Jones in the Ohio Senate when Jones left to run for another seat on the county commission.
He filed to run against Grossmann after retired banker Steve Wilson was appointed to replace Jones.
As four years, ago, Grossmann said he beat his opponent in newer neighborhoods, including those near where Maag lives and had represented.
“That’s what happened with Ariss. I won in newer neighborhoods. He was much closer in older neighborhoods,” said Grossmann, a lawyer and prosecutor, who served as mayor and councilman in Mason before his election to the county commission in 2014.
Grossmann said he and Maag spent most of election day in the parking lot of the same polling place in the Little Miami school district, appealing to voters as they entered the poll.
Maag, 72, a retired pharmaceutical salesman, ran an aggressive campaign, buying TV and radio advertising in the final weeks.
“It was done at a time I could not adequately respond,” Grossmann said.
Grossmann used mailers to counter what he called “intensely negative false ads.” He said current election law allows candidates to campaign on false information.
Last year, Grossmann found himself on the defensive after suggesting the county use invested reserves, rather than borrowing or taxing, to pay for the county’s new $50 million jail.
In the race, he again found himself on defense after Maag blasted him in a statement read in Maag’s absence at a public event and in literature and TV and radio advertising during the campaign.
Grossmann said the radio and TV ads were broadcast although he presented information refuting the claims.
“They all knew it was false,” he said after the election.
Grossmann answered Maag’s claims in later mailers, while continuing to focus on his political and professional life, the growing county’s financial condition and other accomplishments over the past four years he has been in the county office.
“I had a positive campaign. I presented the truth,” Grossmann said.