The lawyer gave Johnson the name of a therapist, whose notes, which the Post reviewed, reference an unspecified event during the campaign that had left her distraught.
In a statement, White House press secretary Sarah Sanders dismissed Johnson's allegation as "absurd on its face."
"This never happened and is directly contradicted by multiple highly credible eye witness accounts," she wrote.
Two Trump supporters that Johnson identified as witnesses - a campaign official and Pam Bondi, then the Florida attorney-general - denied seeing the alleged kiss in interviews with the Post.
While more than a dozen other women have publicly accused Trump of touching them in some inappropriate way, Johnson is the only accuser to come forward since he took office and the only one to allege unwanted contact during the campaign.
Trump faces a defamation lawsuit in New York brought by Summer Zervos, a former Apprentice reality TV contestant, who claims he forcibly kissed and groped her in 2007.
Johnson, an event planner who lives in Madison County, Alabama, is seeking unspecified damages for emotional pain and suffering. The federal lawsuit, filed today in Florida, also alleges that the campaign discriminated against Johnson, who is black, by paying her less than her white male counterparts. A campaign spokeswoman, Kayleigh McEnany, rejected that claim as "off-base and unfounded."
The Post first contacted Johnson nearly a year ago, while reporting on misconduct allegations against Trump, but she declined to comment. In recent days, Johnson's lawyer gave the Post a draft copy of her complaint.
Johnson said she began to consider coming forward in October 2016, after video surfaced of Trump bragging about kissing and groping women without their consent. That was the moment, she said, when she came to view the kiss as part of a pattern of Trump doing whatever he pleased to women.
She said she was nervous about speaking out but had come to regret having worked on the campaign. "I've tried to let it go," she said, beginning to cry. "You want to move on with your life. I don't sleep. I wake up at 4 in the morning looking at the news. I feel guilty. The only thing I did was show up for work one day."
She said she talked to a few other lawyers as she considered her options before, in June of last year, finally hiring Hassan Zavareei, the Washington lawyer bringing the lawsuit. Three months later, she moved to seal a years-old court case in which two family members had briefly sought a temporary restraining order against her. The family members joined her request to have the records sealed, documents show.
Johnson, a 43-year-old mother of four, does not have a long history of political activism. She registered as a Democrat in California several years ago. She said she voted for Barack Obama in 2008 and 2012 but thought Trump might be able to use his business experience to help struggling black communities.
Johnson got interested in the Trump campaign through her stepfather, Jacob Savage, a retired microbiology professor who said he has been active in Republican politics for decades. She met Trump at a November 2015 rally in Birmingham, Alabama, where Johnson said the candidate looked her up and down. "Oh, beautiful, beautiful, fantastic," he said, according to the lawsuit.
She said she looked past the comment and, two months later, took a job as the campaign's director of outreach and coalitions in Alabama. Johnson said she thought she could put her background in human resources and event planning to use on a political campaign.
For the three months before the general election, Johnson was assigned to Florida. Her main responsibility was managing the recreational vehicles that traversed the state as mobile campaign offices. It was inside one, on a rainy afternoon in Tampa, where Johnson said the candidate pressed his lips against hers.
Wearing a dark suit and red tie, carrying an umbrella, Trump walked up to the RV as Johnson stood back and took video. "Good job, boss," she said as he greeted supporters, according to footage she provided to the Post.
Johnson brought volunteers into the RV to take pictures with Trump. She noticed that Trump was attempting to make eye contact with her, she said in the interviews and lawsuit. When it was time for the rally, Johnson said Trump passed her as he exited the RV.
"I've been on the road for you since March, away from my family," she told him, according to the lawsuit. "You're doing an awesome job. Go in there and kick ass."
Trump grasped her hand, thanked her for her work and leaned in, she said.
"Oh, my God, I think he's going to kiss me," she said in an interview, describing the moment. "He's coming straight for my lips. So I turn my head, and he kisses me right on corner of my mouth, still holding my hand the entire time. Then he walks on out."
She said she stood there, feeling humiliated, and Bondi gave her a smile as she walked out of the RV. Karen Giorno, director of the Florida campaign, grabbed Johnson's elbow and gave it a tug, Johnson said in the interviews and lawsuit.
Bondi and Giorno said they do not recall seeing Trump kiss Johnson. They denied reacting the way Johnson described.
"Do I recall seeing anything inappropriate? One hundred percent no," Bondi said in an interview. "I'm a prosecutor, and if I saw something inappropriate, I would have said something."
Giorno dismissed the allegation as "ridiculous," saying "that absolutely did not happen."
Sanders urged the Post to speak with Stephanie Grisham, a spokeswoman for first lady Melania Trump. Grisham, who was Trump's press director in 2016, said she did not see the alleged kiss and was in front of Trump as he exited the vehicle.
Later that day, Johnson called Miguel Rego, her boyfriend of several years. He, too, was working on the campaign in Florida. "I thought it was crazy that he had kissed her. I didn't know how to process it," said Rego, recalling the conversation.
Then Johnson called Savage, her stepfather. "I felt it was a betrayal of trust," Savage said. "I felt I was responsible because, had I not introduced her to the campaign, she would not have been in that position."
Johnson also discussed the incident with her mother, Anne Savage. "She was hysterical," Savage said.
Johnson, however, continued working for Trump, even after an opportunity to work in the campaign's New York headquarters was offered and abruptly rescinded in mid-September, according to her and campaign officials. The position was never filled.
About six weeks after the alleged kiss, on October 7, 2016, the Post published the videotape of Trump boasting about his sexual aggression to an Access Hollywood host. "You know I'm automatically attracted to beautiful - I just start kissing them," Trump said in 2005. "It's like a magnet. Just kiss. I don't even wait. And when you're a star, they let you do it. You can do anything."
Johnson said she was stunned.
"I felt sick to my stomach," she said. "That was what he did to me."
Johnson said she stopped going into the office and, about three weeks before the election, she quit. "She is having nightmares because of what happened," therapist Lisheyna Hurvitz wrote on October 27, according to notes that Johnson obtained and provided to the Post.
Johnson also was talking to lawyer Adam Horowitz, who represents sexual abuse victims, including children. "I believe you and want to see you gain justice and expose this behavior," Horowitz wrote to Johnson in a text dated October 28, 2016. "Right now my practice simply cannot dive into something like that which would be so time-consuming with an uncertain outcome."
She said she once again tried to put the event behind her and even attended one of the inaugural balls. She also twice applied for jobs in the Administration. She said she felt she had earned those opportunities through her work on the campaign. Johnson said that, while she was disappointed, being passed over for those jobs had no bearing on her decision to sue.
Johnson said she grew agitated as the #MeToo movement emboldened women to speak up about sexual misconduct. She said she was also motivated to act as she saw the impact of the president's policies, specifically the detention of immigrant children. "Babies in cages - I didn't think it was going to be this bad," she said.
