In 2371, he was engaged to marry Starfleet Captain Kathryn Janeway. (VOY: "Caretaker") 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.
In early 2374, Johnson married a woman who worked with him. He was relieved when it was revealed that Voyager hadn't been destroyed and was in fact trapped in the Delta Quadrant. He sent a letter to Kathryn 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. (VOY: "Hunters")
Mark Johnson was played by Stan Ivar in "Caretaker" and "Persistence of Vision". His surname was never said aloud on screen, but it can be read from the "Dear John" letter Kathryn received in "Hunters".
The Jeri Taylor novel Mosaic contains additional information about Mark, namely that 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".