He is known for playing Chachi Arcola on the sitcom Happy Days (1977–1984) and its spin-off Joanie Loves Chachi (1982–1983), the title character on the sitcom Charles in Charge (1984–1990), Dr. Jack Stewart in the medical-mystery-drama series Diagnosis: Murder (1993–1995), and the title role of the musical film Bugsy Malone (1976), his onscreen debut.

Baio has guest-starred on various television programs, appeared in several independent films, and starred on the Nickelodeon sitcom See Dad Run (2012–2014).

[6] In 1976, Baio played the title character Bugsy Malone in the children's gangster musical directed by Alan Parker, which also co-starred Jodie Foster.

During his time on Happy Days, Baio earned two Daytime Emmy Award nominations, for his lead performances in the television movies Stoned (1981) and All the Kids Do It (1985).

From March 1982 to May 1983, Baio starred with his Happy Days co-star Erin Moran in the show's spin-off, Joanie Loves Chachi.

During this period, Baio, who played an aspiring singer on Joanie Loves Chachi, was signed to a record contract with RCA Victor, who used his TV role to push his music career.

Baio was a guest-star on a variety of series, including Full House, Touched by an Angel, Veronica's Closet and The Nanny.

[28] On December 15, 2016, Baio accused Nancy Mack, wife of Chad Smith, drummer for Red Hot Chili Peppers, of physically assaulting him at their children's elementary school function.

Baio claims Mack began berating and cursing him over his support of Trump and at one point attacked him, grabbing him under his arms and then shaking and pushing him.

In 2013, Eggert stated on the Nik Richie Radio show that she and Baio had a one-time encounter "years" after Charles in Charge ended, which would have been after 1990.

In the more elaborated account, discussed on The Dr. Oz Show and Megyn Kelly Today, Eggert claimed that Baio molested her beginning in 1986 when she was 14, and that when she was 17, she had intercourse with him.

[32][33][34] Several months later, Alexander Polinsky, another co-star in Charles in Charge, accused Baio of verbally abusing and physically assaulting him while on set.