Both of her parents served in positions of command in Starfleet: her mother (and later captain), Carol Freeman, attempted to keep a close eye on her activities aboard the USS Cerritos, while her father was a Starfleet admiral.
Mariner prided her ability to think for herself in critical situations, as she had served aboard five different Starfleet vessels, including the USS Quito, and had "seen stuff". There was a time when she was interested in advancing in rank, though her views regarding promotion and the importance of the senior staff had become somewhat jaded by 2380. (LD: "Second Contact", "Cupid's Errant Arrow")
On one harrowing occasion, she was trapped in a sentient cave for weeks, describing it as "a dark place that knows things." She also noted that she had almost been decapitated by a singing crystal, and held in a Klingon prison where she had to fight a yeti for her shoes merely because the yeti wanted to be antagonistic. (LD: "Envoys")
She kept additional battle scars from a fight with a tentacled man that stabbed her with a barnacle blade in the chest, a fight on Magus III that left a scar on her right shoulder, a scar on her left bicep from Nanibia Prime, and a scar on her hip from a fight in Scottsdale. After Commander Jack Ransom stabbed her left foot with a crystal sword on Gelrak V, she intended to allow the skin to heal into a new scar as well. (LD: "Temporal Edict")
At some point, she befriended and blood bonded with General K'orin. The two had worked together on Jaxxa Prime, Vulcan, and Earth. Years later, she recalled that she and K'orin had done some "off-the-books grey-ops stuff back in the day." (LD: "Envoys")
Mariner excelled at Starfleet Academy, getting the best grades in her class. She was often noted as being a bit of a role model for her class, and that she always made sure her class kept up with her. Many, such as her friend and classmate Amina Ramsey predicted that Mariner would be the first of her class to get promoted to captain. At some point in this time she was mentored by her mother's old friend William T. Riker, who also rescued her from an alien, though Mariner insists this does not make them "even" due to some unmentioned disagreement. The two even seem to be on first-name basis in the present. (LD: "No Small Parts")
Deep Space 9
Sometime between 2372 and 2375 Mariner started serving aboard Deep Space 9. During this time, she once broke Worf's mek'leth, and, apparently, had it mended before he noticed it was gone. (LD: "We'll Always Have Tom Paris")
Also sometime during the 2370s, Mariner served aboard the starship Atlantis, when nearly half of the ship's crew succumbed to a Kerplickian lice infestation. Since Starfleet was embarrassed by the situation, it was never logged in official records. Mariner took advantage of that fact when she started a rumor that she was a specially trained black ops spy who had killed those who had otherwise simply and officially had mysteriously disappeared.
In 2381, Honus, a Cerritos bartender, shared these rumors about what happened aboard the Atlantis with Ensigns Brad Boimler and Sam Rutherford which caused them to briefly second guess their relationship with Mariner. (LD: "Mugato, Gumato")
While aboard the Quito, she participated in establishing first contact with the Galardonian High Council in 2379. Sometime during 2370 (after Stardate 47025.4) she experienced her good friend and crew mate, Angie, get eaten alive by a Harvongian while their ship was docked at Deep Space 9. The experience was traumatic, causing her to be on high alert in 2380 when she thought something similar could be happening to her friend Bradward Boimler. (LD: "Cupid's Errant Arrow")
During her time aboard the Quito, she spent time in the brig on many occasions and was eventually demoted to ensign and transferred to the Cerritos by her father, under the command of her mother, Captain Carol Freeman. Carol agreed to the transfer under the condition that if their daughter didn't fit in with the crew, her father would transfer her back to the Quito. (LD: "Second Contact")
The Cerritos was Mariner's fifth ship posting. Her fellow command division officer and eventual friend Brad Boimler noted that she hated to give the orientation liaison tour, but offered to provide a tour to Ensign D'Vana Tendi to get her to help moving shore leave contraband. By this time, she had been serving on the Cerritos for a year.
In 2380, while supporting second contact efforts with the Galardonians, Captain Freeman asked Ensign Boimler to monitor Mariner's activities. He eventually believed he had caught her selling Federation weapons, but she had actually been providing needed farming equipment to local farmers in an effort to bypass Starfleet bureaucracy. (LD: "Second Contact")
Aboard the Cerritos, Mariner's assigned bunk was A12001, in the residential hallway used for lower deck officers on beta shift. (LD: "Envoys", "Terminal Provocations") For a brief time, she received another promotion to lieutenant and, as such, received her own quarters, before being demoted back to ensign. Her promotion was actually a scheme to get her to resign to another ship after she disrespected Captain Freeman in front of Captain Durango. (LD: "Moist Vessel")
Mariner was highly proficient at hand-to-hand combat, and was a master of anbo-jyutsu. She was well-practiced at martial arts because it could be done alone and she often lacked friends at her postings. (LD: "Mugato, Gumato")
Mariner had taken two and a half years of horseback riding lessons, though this did not help her in 2381 when she unsuccessfully attempted to ride a horse within Starfleet's Old West Planet holographic training drill. (LD: "I, Excretus")
Mariner claimed to hate her Andorian crewmate Jennifer Sh'reyan and was particularly annoyed by Andy Billups calling her by Jen's name. (LD: "No Small Parts", "Strange Energies") However, she later came to like Jennifer, after Jennifer saved her life in 2381. (LD: "First First Contact")
Mariner's relationship with her mother was often rocky, and conflicting to the point where both of them hoped to keep their relationship as mother and daughter a secret from the rest of the crew. Mariner did not want to be seen as any different from the rest of the crew, and Freeman didn't wish for it to be known that one of the most rebellious officers of the fleet was her daughter. They managed to keep this relationship secret, until Boimler stumbled upon the secret by accident, and inadvertently revealed the relationship to the crew of the Cerritos via an open communications channel. (LD: "Crisis Point", "No Small Parts")
Freeman's patience with Mariner was very often tested, and in 2380, she sought numerous ways to get her transferred off the ship, though none of these methods was successful, and Mariner even caught on that Freeman was trying to have her transferred.(LD: "Second Contact", "Moist Vessel")
Despite this conflicting relationship, Mariner still loved her mother, and when the two cooperated together, they could accomplish impressive feats together. At one point, Freeman had Mariner briefly promoted, initially with the hopes of transferring her off the Cerritos, but with how effective the two worked together in a crisis, she wondered if Mariner would actually be a good addition to her senior staff. While Mariner ultimately got herself demoted again, the two would later agree to work together to provide a positive change in the ways Mariner was good at, but was Freeman was limited to do under Starfleet protocol. (LD: "Moist Vessel", "No Small Parts")
While Mariner was not above taunting Freeman often, and giving her a piece of her mind, the two would learn to see past their differences and their quarrels, and find a new respect and love for one another. While the two didn't always understand one another, their conflicts never hurt their love for one another, and Mariner logged that as much as Freeman could be a "pain in the ass", she would do anything for her. (LD: "Crisis Point", "I, Excretus")
Mariner considered Boimler her best friend, and the two often worked together on missions. Though their friendship started a bit rough, and though their personalities and approach to their missions could be drastically different, with Boimler being more by the book, while Mariner would act more on instinct, they worked well together, and developed enormous amounts of respect for one another. As a show of friendship, Mariner decided to become Boimler's mentor with the intention of getting him into a captain's chair. She also helped restore some faith in himself as a Starfleet officer by deliberately making herself look bad when he got close to quitting Starfleet. (LD: "Second Contact", "Envoys", "Strange Energies")
When Boimler accepted a promotion, and transfer to the USS Titan, Mariner was very angry and hurt that he didn't say goodbye, and held a grudge against him for a brief period of time. While she missed him, she did eventually understand his decision, and was happy for him. Even three months after the transfer, Mariner, Tendi, and Rutherford would keep up the habit of leaving an open booth for Boimler when using the sonic showers due to his discomfort with public nudity.
Boimler's time on the Titan was limited, however, and when he returned to the Cerritos, Mariner was very excited to see him return, and welcomed him back instantly (though she wasn't above giving him a little flack for leaving). While she welcomed him back, she did hold a small bit of anger at him, which was revealed during a visit with Boimler to Starbase 25. While Mariner was angry with him for never apologizing, Boimler was angry that she was being selfish, and that he earned his promotion to the Titan. The two eventually reconciled, with Boimler giving her the apology she had been waiting for. (LD: "No Small Parts", "Strange Energies", "Kayshon, His Eyes Open", "An Embarrassment Of Dooplers")
Despite the arguments Mariner and Boimler could often find themselves in, the two were practically inseparable to the point where Tendi did not dismiss the assumption of them to be dating. They affectionately teased one another, and Mariner considered Boimler her "number one". (LD: "We'll Always Have Tom Paris", "An Embarrassment Of Dooplers")
Mariner and Tendi were friends, but did not have a very close relationship for a while. Since Tendi was closer with Rutherford more often than not, for the first year that Tendi served aboard the Cerritos, Mariner was completely unaware that Tendi even had a first name. Tendi was also unaware of a lot of Mariner's service history with Starfleet. Mariner also had a habit of associating Tendi with common stereotypes associated with Orions, much to the discomfort and frustration of Tendi, who was trying to distance herself from such stigmas. (LD: "Crisis Point", "We'll Always Have Tom Paris")
Despite this, the two did grow closer as friends as time went on to the point where in a single mission, Tendi disclosed more information about herself than even Rutherford knew. Mariner saw Tendi as a very kind and good person, and was willing to take the blame for her when she'd get into trouble.
William T. Riker
Aboard the Cerritos, Mariner went on one date with Lieutenant Steve Levy in 2379, but ended it because he believed conspiracy theories, such as that "Wolf 359 was an inside job." (LD: "No Small Parts")
- LD: Every episode
Mariner was voiced by Tawny Newsome.
In developing the characters for Lower Decks, Mike McMahan described all four leads in an 10 August 2020 TrekMovie.com interview as a combination of other Star Trek characters, parts of himself, and people in his life. Mariner had elements of Captain James T. Kirk, Kathryn Janeway, and Jean-Luc Picard, "she knows the rules, but also sometimes feels like she should bend them because she knows better." He further compares her to "Maverick in Top Gun... he's great at flying a jet, but he also buzzes the tower." 
McMahan named Mariner after his sister, Beckett Mariner McMahan, and based her relationship with Captain Freeman on his sister's relationship with their mother "when my sister was younger and would throw down with my mom occasionally."  
No explanation is given for Mariner having a different surname than both of her parents.
Beckett Mariner appears as a duty officer in Star Trek Online.