Thanks to a lawsuit filed by West Marin native Jenn Nelson, the “Happy Birthday” song has been set free. A federal judge last week ruled that the lyrics to the famous song have not been subject to copyright since 1935—though corporations have collected millions in royalties for its use since then.

“Ultimately, the happy birthday song is the people’s song and it belongs to the people and that’s where it needs to be,” said Ms. Nelson, a filmmaker who grew up in Point Reyes Station. She left West Marin at 25, and now lives in New York City. The idea for an investigation into the rights to the song began when she was working for MTV’s reality show My Sweet Sixteen. “I spent a lot of time filming these ridiculous over-the-top birthday parties. Kids would get two cars or an elephant,” she said.

But what seemed just as bizarre were the high royalty fees that the station paid to Warner Music Group, which has been said to make millions from royalty payments since it purchased rights to the song in 1988. Ms. Nelson decided to begin working on a documentary about the song, for which she was charged $1,500 to use.. The melody—written by sisters Mildred and Patty Hill—first appeared in print in 1983, with different lyrics. But the authorship of the birthday lyrics has a hazier history, though Summy Corporation, the sisters’ publisher, asserted a copyright claim in 1935. The lawsuit, which involved Ms. Nelson and three other plaintiffs, made a number of contentions about the copyright holding. In his final ruling, federal judge George King said that the copyright issued in 1935 was only for particular arrangements of the song, meaning that no copyright holders since that year have had a legal right to the lyrics. Ms. Nelson doesn’t know if there will be an appeal, or when her documentary will be finished. But the process of making the film has certainly changed its ending.