Mark Strong Sr. was a full partner who controlled, supervised and managed the prostitution business being run by the fitness instructor, prosecutors said.
Strong, 57, acknowledged having an affair with Alexis Wright and helping her open the studio but contended he didn't profit from her activities. His attorneys said he was a smitten lover who wanted to help the single mother.
Wright is awaiting trial and has pleaded not guilty.
Jurors deliberated for 4 1/2 hours before announcing that they had found Strong guilty of all 13 counts - 12 of promoting prostitution and one of conspiracy. He showed no emotion as the verdict was announced. His wife leaned into his son and cried.
The scandal in Kennebunk, a seaside village known more for its sea captains' homes and beaches than crime, attracted international attention last fall after it was reported that Wright's ledgers indicated she had more than 150 clients and made $150,000 over 18 months.
Strong contends he co-signed for Wright's studio and loaned her money that was repaid. But the defense said he never recruited johns or profited from Wright's business.
Testimony and videos presented to jurors indicated Strong was familiar with operational details of Wright's prostitution, chatting via Skype before and after her appointments and watching the sexual encounters from his office 100 miles up the coast in Thomaston. Before each tryst, Wright took time to make sure the Skype video camera was hidden and pointed at the massage table where the encounters took place.
Law enforcement officials said the prostitution took place at Wright's home, office and Zumba studio.
But all of the evidence presented during the trial focused on her rented office, where law enforcement officers seized video equipment, computers, condoms and other items.
Even as lurid details emerged in the courtroom, Strong's wife of 30 years and several other family members remained seated several rows behind him to show support.
One thing that was missing from the trial was testimony from Wright's accused clients. Eighteen of them were on the state's list of witnesses, but none of them testified after attorneys stipulated that the encounters took place.
The judge previously dismissed 46 invasion-of-privacy counts that stemmed from videotaping of prostitution clients without their knowledge.
The prostitution charges and ensuing publicity came as a shock in Kennebunk, a town of 10,000 that borders Kennebunkport, home to the Bush family's Walker's Point summer compound.
The verdict in the delay-plagued trial came more than six weeks after the start of jury selection, which was halted twice because of legal action that went to the state supreme court, leaving potential jurors in limbo for weeks.