In 2371, he was engaged to marry Starfleet Captain Kathryn Janeway. (VOY: "Caretaker", "The Disease") As a gift to mark their engagement, he gave Kathryn a copy of Dante's Inferno. (VOY: "Shattered") Following the loss of the USS Voyager, Johnson was devastated, but he held out hope that Janeway and the rest of the crew were alive longer than most people did. Eventually he realized that he was clinging to a fantasy. He began living his life again, meeting people and letting go of the past.
When it became apparent that on-ship romances were becoming more and more prevalent aboard Voyager, Janeway mentioned to Chakotay that that was a luxury she couldn't have as ship's captain, adding, "besides, I intend for us to be home before, before Mark gives me up for dead." (VOY: "Elogium")
Janeway later confessed to Kes that she would occasionally "dream about being with Mark and it's so real. Then when I wake up and realize it's just a dream, I'm terribly discouraged. In those moments, it's impossible to deny just how far away he really is. And I know that someday I may have to accept that he's not part of my life anymore." (VOY: "Tuvix")
In 2372, Q salaciously encouraged Janeway to forget Mark in favor of him, as part of an attempt to persuade her to rule in the Q Continuum's favor during Quinn's political asylum proceedings. (VOY: "Death Wish")
Janeway had several letters home that she started in 2373, including one to her family, and one to Mark. After the promise of contact with the Alpha Quadrant was looking to be a reality, later the following year, she decided to make a few updates to the letters, just in case. (VOY: "Message in a Bottle")
In early 2374, six months after Starfleet Command had declared Voyager lost with all hands, Johnson married a female coworker. He was relieved when it was revealed four years later that Voyager hadn't been destroyed and was in fact trapped in the Delta Quadrant. He sent a letter to Janeway, telling her what had happened in his life and that he was sorry. He additionally told her that her dog Mollie had given birth to a litter, and that he found homes for most of the puppies, and the others got along well with his kitten. Although saddened, Janeway did not blame him and logically had expected that he would have moved on by then. (VOY: "Hunters")
In 2375, a falsified letter transmitted to Janeway by an alien lifeform was supposedly from Johnson. This letter had him being engaged rather than married and stated that the engagement had broken off. (VOY: "Bliss")
Mark Johnson was played by Stan Ivar.
His surname was never said aloud on screen, but it can be read from the "Dear John" letter Kathryn received in "Hunters".
In the first draft script of "Caretaker", Mark was described as "a civilian man [...] who is used to this flurry of activity from the woman he loves."
The novel Mosaic, (at one point considered canon by some in production,) contains additional details of Johnson's background. It notes he was born in the year 2333 in Bloomington, Indiana and raised by traditionalist parents in an agricultural community. It goes on to reveal that when he was six, he lost a pet dog and he had a very difficult time getting over it, and while a youth, he went by the name "Hobbes" (his full name being Mark Hobbes William Johnson). Furthermore, it states that he attended The Meadows school and when he was nine, he began taking tennis lessons with Epkowicz, but refused to quit even though Epkowicz told him to. Two years later, he was taking lessons with coach Cameron, who also coached Kathryn Janeway. In 2349, Johnson was swimming with Kathryn Janeway in a cave system under Olympus Mons on Mars. When trying to find an entrance to an underground cave, Johnson was pulled into a trench and Janeway saved his life by helping him get out. In 2351, Johnson began attending Indiana University. He still played tennis occasionally, but he was a part of the school's prestigious swim team. He graduated from Indiana with a degree in philosophy. By 2359, the novel explains that Johnson was part of a respected philosophical symposium based in South America called the Questor Group. By then, he had decided to switch to using his first name, "Mark".
In an alternate timeline featured in the Star Trek: Myriad Universes novella A Gutted World, Voyager was never stranded in the Delta Quadrant and Janeway and Mark were married by 2373. He was widowed when the ship was destroyed in that year by the Cardassians.