Bilbo Baggins was born on September 22, 2890 of the Third Age, the son of Bungo Baggins and Belladonna Took. In 2941, he joined Gandalf, Thorin Oakenshield and his twelve Dwarves on the quest to reclaim the Lonely Mountain from the dragon Smaug. This led to an adventure which took Bilbo and his companions to Rivendell and through Mirkwood, eventually reaching the Lonely Mountain. Here, after the mountain was reclaimed by Thorin Oakenshield, the Battle of Five Armies took place.

During his adventures in The Hobbit, Bilbo also found the One Ring, and escaped from Gollum when he won a riddle competition with the question "What have I got in my pocket?" However, during the contest, he also revealed the names of "Baggins" and "The Shire" to Gollum. Bilbo was the Ring-bearer for many decades with no idea of its significance, and it prolonged his life beyond normal limits and slowed his aging. He used this ring to its fullest ability when the expedition was captured by spiders in Mirkwood, and also when Thorin and his companions were imprisoned by the Mirkwood Elves. At the Battle of Five Armies, Bilbo used the ring to prevent himself getting hurt. This meant it was difficult to find him after the battle, but eventually he regained consciousness and took the ring off himself. He further remarked to Frodo in The Lord of the Rings that it was useful for hiding from the Sackville-Bagginses when they came to visit.

In The Lord of the Rings, Bilbo left the Shire on the day of his eleventy-first (111th) birthday, (September 22, 3001), leaving the Ring and all the rest of his estate, including his home Bag End, to his relative and heir Frodo. His disappearance at his party caused him to be later remembered as "Mad Baggins". He traveled to Rivendell, accompanied by three dwarves, where he lived a very pleasant life of retirement: eating, sleeping, writing poetry, and working on his memoir, There and Back Again, known to us as The Hobbit. He also wrote a book called Translations from the Elvish, which formed the basis of what is known to us as The Silmarillion.
At the end of The Return of the King, Bilbo accompanied Frodo Baggins, Gandalf, and Elves to the Grey Havens, there to take ship for Valinor across the sea, on September 29, 3021. He had already celebrated his 131st birthday, becoming the oldest Hobbit in Middle-earth.