Her career began in 2000 when she was 18. Her first film was Real Sex Magazine. As the scene involved anal sex she had to perform her first enema, and she was also required to urinate on her co-star. One of her early films featured Sinclair in a rough group sex scene with twelve men, staged in a prison. In this scene she was choked, slapped and verbally abused and often appeared to be on the verge of tears. She has since appeared in over 300 adult films, including titles such as Service Animals 6 & 7, She-Male Domination Nation, Bella Loves Jenna, Back 2 Evil, Weapons of Ass Destruction, and Fashionistas Safado: The Challenge. In 2002 she featured in the multi award-winning BDSM themed Fashionistas; its Director John Stagliano described her as "a woman with the most incredible sexual abilities I’ve ever seen." As well as Stagliano she has often been directed by Nacho Vidal, Jules Jordan and Rocco Siffredi. In 2003 she signed a contract with Sineplex, for whom she also directed.

Her career in the pornography Business was followed for two years by a crew from ABC Television, culminating in a January 2003 interview with Diane Sawyer as part of a Primetime Thursday segment on pornography. At one point in the interview Belladonna broke down in tears, when Sawyer asked her, "You keep describing these awful things that happened to you. Yet, you keep smiling. Why?" Belladonna's smile wavered and her eyes watered up. "It's so I don't start crying," she said. Part of the footage for the segment was an interview with Belladonna's sister, who had moved out to Los Angeles at Belladonna's urging, and who had gotten a job at Red Light District Films. Belladonna's sister revealed on-camera that despite her claims, Belladonna was actually quite depressed, and had recently been hospitalized following a suicide attempt. Another topic discussed in the interview was the fact that Belladonna had contracted chlamydia. The publicity provided by the interview boosted Belladonna's career.

She has appeared several times in the reality television series Family Business, which focuses on the life of porn Director Adam Glasser a.k.a. "Seymore Butts". In 2004 she was one of a number of porn stars photographed by Timothy Greenfield-Sanders for a "coffee table book" on the adult industry, XXX: 30 Porn-Star Portraits; she also appears in the HBO documentary Thinking XXX about the making of the book.

The Fans of Adult Media and Entertainment Awards (F.A.M.E.) were created in 2006 by Genesis magazine, Adam & Eve Sex Toys, WantedList.com and AVN as a means for the public to vote for their favorites.

In August 2007, Sinclair decided to semi-retire from performing with other people over concerns of STDs, specifically herpes. She thought she had contracted herpes in 2002 and was worried the disease had spread; however it was later discovered that it was a skin rash rather than herpes. Regardless, she still decided to semi-retire.

In 2008, she came out of retirement, and has filmed a number of hardcore scenes. Her last adult film was Odd Jobs 4 in 2008, in which she was in a gardening scene. She appeared in Digital Playground's Pirates II, the sequel to their top-selling DVD, Pirates. She has also appeared recurringly in James Gunn's PG Porn.

In June 2010 it was announced that she would be part of the cast of the upcoming horror movie Stripped.

The AVN Awards are sponsored and presented by the American adult video industry trade magazine AVN (Adult Video News). Belladona has been nominated for 43 AVN Awards, of which she has won 10. In 2011, she was inducted into the AVN Hall of Fame.

In July 2012, she announced via Twitter that she was "no longer interested in having sex on camera" and would be pursuing her other interests.

In 2014, Sinclair appeared briefly in Paul Thomas Anderson's Inherent Vice as the sister of a biker.