McCartney has had an extensive voice acting career, voicing characters such as Theodore in the Alvin and the Chipmunks film series, JoJo McDodd in Horton Hears a Who!, Dick Grayson in Young Justice, and Roxas and Ventus in the Kingdom Hearts video game series.

At fifteen, he began work on a solo career with a local band, featuring musicians Dillon Kondor (guitar), Peter Chema (bass), Katie Spencer (keyboards), Alex Russeku (drums), Karina LaGravinese (background vocals), Sharisse Francisco (background vocals), and under the management of Ginger McCartney and Sherry Goffin Kondor, who co-produced his first album Beautiful Soul.

In 2004, he performed a duet with Anne Hathaway, "Don't Go Breaking My Heart", which is featured on the Ella Enchanted soundtrack.

McCartney promoted Departure on a co-headlining tour with Jordin Sparks, which began in August 2008 and ended in September.

On December 3, 2010, McCartney announced through his Facebook page that he pushed the release date for the album back to early 2011.

On April 7, 2011, McCartney responded to a question on his Twitter page about the delay, saying it hinged on pickup of Locke & Key.

"[19] On May 6, 2012, McCartney's mother said on Twitter that the replacement of the President of his label, Hollywood Records, in January 2012 was delaying the release of his album.

[20] In the March 2013 issue of Glamouholic magazine that he has covered, an exclusive interview was conducted and he confirmed the release of his anticipated fourth studio album, after all the disbandments of his record label, within this year.

On May 13, it was confirmed on On Air with Ryan Seacrest that McCartney would be joining the Backstreet Boys and DJ Pauly D on their In a World Like This Tour, which kicked off on August 2, 2013.

On August 13, the singer released "Back Together", the lead single from his fifth studio album, published by the independent label Eight0Eight Records.

The official music video featured former Greek castmates Scott Michael Foster, Jacob Zachar, and Paul James.

[31] On June 11, 2020, on what would be Trousdale's 35th birthday, McCartney and his former Dream Street bandmates Greg Raposo, Frankie Galasso, and Matt Ballinger reunited for a virtual performance of "It Happens Every Time" to pay tribute to him.

With "racy lyrics and familiar early '00s pop sounds", the record was preceded by two singles: "Faux Fur" and "Make a Baby" featuring Yung Gravy.

McCartney went on tour in promotion of the album, beginning on April 12 in Austin, Texas; its final show is slated for May 16 in Los Angeles.

[38] He also later starred in the short-lived series Summerland (2004–05), which aired on The WB for two seasons, playing orphaned teenager Bradin Westerly.

In 2008, McCartney co-starred along with Elisabeth Harnois in an independent teenage drama feature film, Keith, directed by Todd Kessler.

According to Entertainment Weekly, in December 2008, McCartney was negotiating to play against type in the role of the Fire Nation's Prince Zuko in M. Night Shyamalan's feature film adaptation of Avatar: The Last Airbender.

[40] In February 2009, British actor Dev Patel replaced McCartney, whose tour dates conflicted with a boot camp scheduled for the cast to train in martial arts.

[42] McCartney appeared as a recurring character in the ABC Family series Greek for several episodes playing a star football talent who joins Kappa Tau.

His character eventually decides to depledge the fraternity after citing pressures between football and Greek life combined with Rusty stealing his girlfriend Jordan.

He also provided the voice work for Ventus, a character in the video game of the same series Kingdom Hearts Birth by Sleep in 2010.

He has been added to the Season 7 (2013) cast of the Lifetime Network series Army Wives, portraying a young soldier deployed to Afghanistan with an 18-year-old wife at home.

Later in 2005, McCartney signed on as an official supporter of Little Kids Rock,[46] a nonprofit organization that provides free musical instruments and instruction to children in underserved public schools throughout the U.S.A.

He has donated proceeds of his 2005 tour for disaster relief, recorded radio spots promoting the "Kids For A Drug-Free America" campaign, is a spokesperson for the St. Jude Children's Research Hospital, and is involved in the charity SPACE, which was co-founded by a childhood friend of his mother's.