Mariner prided her ability to think for herself in critical situations, having noted "I have seen stuff" and having served aboard five different Starfleet vessels, including the USS Quito. 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.
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. (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. (LD: "Much Ado About Boimler")
While aboard the Quito, she participated in establishing first contact with the Galardonian High Council in 2379. Sometime in the 2370s she experienced her good friend and crew mate, Angie, get eaten alive by a Harvongian shape-changer 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. During her tour aboard the Quito, she spent time in the brig on many occasions and was eventually demoted to ensign and transferred to the USS Cerritos by her father, under the command of her mother, Captain Carol Freeman. (LD: "Second Contact", "Cupid's Errant Arrow")
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")
Both Mariner and her mother attempted to conceal their familial relationship from the general public, though Brad Boimler became aware of it by accident, and later revealed it to the crew on an open communications channel. (LD: "Crisis Point", "No Small Parts")
Mariner was good friends with Ensign Sam Rutherford, and closely followed his dating life.
Though Mariner had initially thought Brad Boimler "a boring worker drone" and "an ambitious little weasel", after a year of serving together, he came to believe that "Starfleet [was] better with [her] in it." As a show of friendship, Mariner decided to become Boimler's new mentor with the intention of getting him into a captain's chair. (LD: "Second Contact")
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."  
Beckett Mariner appears as a duty officer in Star Trek Online.