Mablung served as chief Marchwarden of Doriath. He was one of the few Doriathrim allowed to interact with the Noldor. Notably, he and Daeron attended the Feast of Reuniting (Mereth Arderthad). Mablung went on the hunt of the wolf Carcharoth, cut open the wolf's belly to retrieve the Silmaril, and put it in Beren's hand before he died. (This makes him the only one who had touched a Silmaril and not coveted it.) Mablung and Beleg were the only two of Doriath allowed to join the Fifth Battle so long as they did not do so under the banner of a son of Feanor. When Nargothrond fell, he sought for Turin in the ruins and was the only one to survive an attack by Glaurung the Great Worm. He found Nienor and tried to escort her home, but she fled from him. When he finally found Turin, he recounted what had become of Nienor, confirming that Turin had married and impregnated his own sister and causing Turin's suicide.
After Thingol was killed by the dwarves, Melian spoke to none save Mablung, whom she told to go to Beren and Luthien. Mablung was slain while protecting the treasury.
Sauron found Mablung shortly after his death, took him to Angband, and brought him back to life. Sauron took a liking to Mablung because stubborn personality and high tolerance for pain was reminiscent of Maedhros and personally tortured him. Maeglin and Ecthelion met Mablung during their captivity in Angband. He is finally freed by Sauron during one of his alignment swings.
In Gondolin, Mablung befriends Nar because both of them have until recently been prisoners of Angband and because both were more familiar with speaking in Sindarin whereas only Quenya was spoken in Gondolin. Nar was learning archery and tree walking from Duilin so Mablung aligned himself with the House of the Swallow, which suited him well since the Swallows are similar in nature to the marchwardens. He escaped with them down Sirion.
When the sons of Feanor met with the remnants of Gondolin, Sauron's interest shifted wholly to Maedhros. Their shared experiences as Sauron's plaything inevitably pushed them together, and Mablung now serves as Maedhros' bodyguard and lover.
Sauron gave Mablung several artefacts as part of the ongoing mind games: Belthrondig, Beleg's bow, and Anglachel, which was Beleg's sword and initially retrieved by Mablung after Turin suicided upon it (then called Gurthang).
Mablung has only had male lovers, and it was speculated among the Doriathrim that he remained unmarried because the nature of his job required him to be gone from Menegroth for long periods of time. During the days when Thingol was staring into Melian's eyes, Mablung was considered low class and basically had sex with whoever demanded it of him. He did not get a boyfriend after his first one had an accident that was arranged by a jealous nobleman.
Once Thingol returned and restored order to the lands, Mablung became a marchwarden of Beleriand and slept with other marchwardens during their excursions. His preference for men may have come about because of an unfortunate incident in which he caught sight of a naked Luthien and was severely punished by Thingol. His only regular long-time partner was Beleg.
During his time with the Gondolindrim, he commented on Halatir's handsomeness but was rejected. Nevertheless, his interest caused some tension between him and Heledir, who was also interested in Halatir. Mablung was interested in helping Nar develop sexually after his unpleasant experiences in Angband but intended to do so gradually and wait until he was older to actually have sex with him. Before that, Duilin swept in and took Nar.
Mablung took an interest in Menelvagor because they had both suffered in Angband and he was good-looking in a rugged way. Being of equal social class, they had few compunctions about making out. Mablung's relationship with Maedhros took longer to develop because Sauron actively prevented it and their difference in social class. Once Maedhros made it clear that he wasn't obsessed with such delineations and wasn't so great that Sauron couldn't be blamed for wanting him so badly, the sexual tension between Maedhros and Mablung diffused. Shortly after, the two became lovers. Mablung does not have a problem with multiple relationships and is still devoted to both despite bonding with Menelvagor to fight off Sauron's spell. Mablung's shorter height doesn't bother him or make him a less aggressive lover.