Mac Tonight is a character used in a marketing campaign for McDonald's restaurants from the late 1980s to the 2000s. The character was known for his giant crescent moon head, sunglasses, piano playing, and crooner parody of "Mack the Knife", which was made famous in the United States by Bobby Darin. The original campaign's Mac was performed by actor Doug Jones and voiced by Brock Walsh.

The campaign was conceived in 1986 as a local promotion to increase dinner sales for Southern California licensees, and its popularity prompted a nationwide campaign in 1987. In 1989, Bobby Darin's son, Dodd Mitchell Darin, filed a lawsuit against McDonald's for allegedly infringing upon his father's trademark.

Following the lawsuit, McDonald's stopped using the song and mostly retired the campaign. None of the several 1990s reboot attempts were successful, including a NASCAR sponsorship in the late 1990s. A separate animated campaign featuring the character was launched in Southeast Asia in 2007. In the late 2000s, the character was appropriated as "Moon Man", an Internet meme that became associated with white supremacy and the alt-right. The Anti-Defamation League added this racist parody to its database of hate symbols in 2019, and the Department of Homeland Security used the character in its own unauthorized clip in 2025. In the 2010s, the character's image became an icon in the vaporwave music genre.

The campaign, created by Jim Bennedict and Peter Coutroulis, was created for Southern California McDonald's franchisees by Los Angeles advertising firm Davis, Johnson, Mogul & Colombatto, for a budget of around US$500,000 (equivalent to about $1,500,000 in 2025). Looking to increase the dinner business, the agency was inspired by the song "Mack the Knife" by Kurt Weill and Bertolt Brecht, made famous in the United States by Bobby Darin in 1959. The agency listened to different versions of it before opting to create an original version with new lyrics. After deciding not to feature real people, the designers settled on an anthropomorphic crooner moon on a man's body with 1950s-style sunglasses, playing a grand piano atop either a floating cloud or a giant version of the namesake Big Mac sandwich. The song and style were designed to appeal to baby boomers as a revival of 1950s-style music in popular culture, and to garner a "cult-like" following akin to Max Headroom.

From 1986 to 1987, the campaign expanded to other cities on the American West Coast. McDonald's said that the campaign had "great success", while trade magazine Nation's Restaurant News announced that it had contributed to increases of over 10% in dinnertime business at some Californian restaurants. A crowd of 1,500 attended the visit of a costumed character to a Los Angeles McDonald's. With concerns that he was too typical of the West Coast, in February 1987, it was decided that the character would feature on national advertisements, which aired that September. He attracted a crowd of 1,000 in Boca Raton, Florida. A September 1987 survey by Ad Watch found that the number of consumers who recalled McDonald's advertising before any other doubled from the previous month, and was higher than any company since the New Coke launch in 1985.

Doug Jones performed Mac Tonight for 27 out of the 29 commercials from 1986 to 1997. In 2013, he recalled "that's when my career took a turn that I was not expecting. I didn't know that was a career option." Mac Tonight's voice was provided by Brock Walsh. Director Peter Coutroulis, who had won a Clio Award for a previous campaign for Borax, pitched several advertisements which did not air, including a "Spielberg-like" production inspired by E.T., in which two astronomers watch Mac Tonight drive his Cadillac through the sky.

In 1989, Bobby Darin's son Dodd Mitchell Darin alleged that the song infringed upon his father's trademark without prior permission. Darin filed both a lawsuit and an injunction for the song to be removed from both TV and radio ads. As a response to the lawsuit, McDonald's stopped airing the advertisements.

They thought that I had co-opted his father’s singing style, and they filed suit for infringement of likeness. Specifically, my vocalization was apparently the issue. To me though, Bobby Darin wasn’t the imprint on that song. I was more influenced by guys like Frank Sinatra, Ray Charles and Louis Armstrong — Louis Armstrong was known for this song, too.