Born and raised in Hawkins, Indiana, Lonnie married Joyce sometime before 1966, the year they had their oldest son Jonathan, followed by Will in 1971. Lonnie was an implied alcoholic and drug addict who was abusive to his family; he forced his son Jonathan, who was 10 at the time, to shoot a rabbit, and used to call Will a fag. Joyce divorced Lonnie sometime before 1982, when they lost contact and he moved to Indianapolis, Indiana that same year, maintaining little interest in seeing his children. He also began dating his current girlfriend, Cynthia.
In November 1983, Will, now 12, was abducted by The Monster and taken to the Upside Down. In the first episode, "The Vanishing of Will Byers", Joyce had attempted to call Lonnie to inform him that their son is now missing, only to be answered by his much younger girlfriend Cynthia who rudely hung up on her. It was said the police there later questioned Lonnie.
The next day, in the second episode "The Weirdo on Maple Street" Jonathan skipped school to drive to Indianapolis, and entered the run down home of Lonnie and Cynthia. Thinking he was an intruder, Lonnie pushed Jonathan against a wall, before introducing him to Cynthia. While showing Jonathan around to show him that Will's not there, Lonnie claims that he wants to see his sons and tells Jonathan that he sounds like Joyce when it comes to calling Lonnie the bad guy. Jonathan gives his father a missing poster before leaving.
When Martin Brenner and the Hawkins National Laboratory faked Will's death by producing a fake body, and making it look like he had fallen into the quarry and was killed, Lonnie came to Hawkins for the funeral in episode 5, "The Flea and the Acrobat". He tried convincing Joyce to get psychiatric help, and told Jonathan that his mother is sick. Pretending to be in mourning, Joyce later found in Lonnie's wallet a flyer for a lawsuit for accidental death or injury, realizing that Lonnie was only there to sue the quarry's company to get money. An enraged Joyce confronted Lonnie about this, who claimed they could use the money to pay for Jonathan's school, but he didn't know which school he's always wanted to go to. Joyce told Lonnie to leave, before he blamed Joyce for what happened and called her a mess and a bad mother. Lonnie eventually left, and it's unknown what happened to him afterwards.
Lonnie is an outright despicable man, nothing short of a sleaze. He's a drunkard, uncouth and a complete slob. He's extremely selfish, egotistical and is virtually devoid of redeeming qualities. He has no respect for his ex-wife Joyce, and has no love at all for his sensitive second son Will, viewing him as weak and a waste. He's quite aware of how much his family hates him and is more than willing to spite them.
Lonnie was originally planned to have a redemption of sorts, but that idea was scrapped for unknown reasons.
In "The Flea and the Acrobat", Lonnie seals the broken wall of the Byers house. This inadvertently blocks the portal of the house, preventing Will from escaping the Upside Down, an alternate dimension.