Grimes started acting at an early age. In 1984, at the age of 13, he co-starred with Mickey Rooney in a made-for-TV holiday movie entitled It Came Upon the Midnight Clear. In 1985, he appeared in an episode of the Twilight Zone called "Little Boy Lost".

Grimes also began singing in childhood, and in March 1986 appeared on a Bob Hope TV special, singing "Somewhere Over The Rainbow". The special was taped in Sweden with the King and Queen in attendance. In 1987, he performed as a guest vocalist on the Time album by Richard Carpenter of The Carpenters, where Carpenter thanked the 15-year-old Grimes for his "spirited" performance, commenting in the liner notes, "I believe young Scott to have quite a future in music." Grimes' first music album, Scott Grimes, was released by A&M Records in 1989, and produced by Carpenter. In 2005, he released his second album, Livin' on the Run, and in 2010 his third, Drive.

In 1987, Grimes voiced Pinocchio in the dark animated film Pinocchio and the Emperor of the Night. In 1988, he appeared with Dwayne Hickman, Connie Stevens, and her daughter Tricia Leigh Fisher in the TV-movie Bring Me the Head of Dobie Gillis; Grimes, Hickman, Stevens, and Fisher also appeared on the All-Star Super Password Special with Bert Convy. He had also guest starred on the NBC sitcom Wings. He is also known for his 1987 recurring role on Who's the Boss as Alyssa Milano's character's love-interest, Chad McCann and his regular appearances in the 1994–2000 series Party of Five as Will McCorkle.

Grimes appeared on the NBC prime time show ER playing Dr. Archie Morris from 2003 until the series' end in 2009. Initially a recurring character on ER, he later was upgraded to a series regular in Season 12. The character initially provided comic relief, but grew over the years, becoming central to several episodes and story lines in the series' final seasons.

Grimes's film credits include the dark comedy Who's Your Monkey and Ridley Scott's Robin Hood with Russell Crowe and Cate Blanchett. On October 11, 2010, Grimes announced on his Twitter website that he would voice a recurring character on the animated series Family Guy. That recurring character is Joe Swanson's previously thought-to-be-dead son, Kevin Swanson. On March 19, 2013, the actor was cast as Dave Flynn on the NCIS: Los Angeles episode entitled "Red", the first episode of a two-part backdoor pilot to another NCIS spin-off to be named NCIS: Red, which was not picked up. Grimes had previously appeared in an NCIS episode entitled "Baltimore" in 2011 as Detective Danny Price, Anthony DiNozzo's former partner at the Baltimore P.D.

In 2017 he will star in Seth MacFarlane's new show The Orville.

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