Murray will challenge Ed Murray, who defeated him in 2013.
Heckard was identified only by his initials in the original lawsuit filed earlier this month against Murray, but the lawyer filed an amended lawsuit on Tuesday naming Heckard.
McGinn, who was Seattle's mayor from 2010 to 2013, is pledging to reform Seattle tax policy, which has been called the most regressive system in the nation. Murray has forcefully denied the allegations.
Murray says he'll run for a second term. In 1984, the state Legislature approved a law that prohibits a county, city or city-county from levying a tax on net income.
Heckard is described as an openly gay man with no real political inclinations, which contrasts with the mayor's claims that the allegations are a part of a right-wing conspiracy because the Mayor pushed the LGBTQ civil rights bill.
McGinn, a former Sierra Club activist and attorney, was an often a combative presence at City Hall who thrilled some but often annoyed Seattle's power structure.
He was also famously opposed to the waterfront tunneling project, a stance for which he was ridiculed at the time. After a two-year delay for repairs on the boring machine, Bertha, tunneling was completed earlier this month. The tunnel is now scheduled to open in 2019, three years behind schedule. The city's downtown businesses supported the panhandler ordinance, saying people felt unsafe.
"It wasn't very long ago that we were anxious about attracting downtown employers", McGinn said.
In a written statement Wednesday, Murray said: "I can state categorically that I have never had a sexual relationship with this individual, Delvonn Heckard, of any kind or at any time". He thinks doing so will prompt people to believe his claims.
Now it would seem, with a lawsuit alleging Murray coerced a minor into sex and no sign he'll step down, McGinn has found an opening for a rematch.
In a press conference at his Greenwood home this morning, McGinn said he's "really trying to avoid talking about the mayor's legal troubles", but that he "looked at the lay of the land" before announcing.