After giving up a boxing career, Hagar began singing in the late '60s, performing with various California bands including Skinny, the Fabulous Catillas, Justice Brothers, and Dust Cloud. During this time, he built up a solid reputation in the California hard rock scene. Former Edgar Winter guitarist Ronnie Montrose asked Hagar to join his band, Montrose, in 1973. Hagar recorded two albums with Montrose before going solo in 1976, taking the group's bassist, Bill Church. Montrose's drummer Denny Carmassi later joined Hagar's band, along with keyboardist Geoff Workman.

Hagar's self-titled "red album" was his first chart entry; it eventually went gold. In 1979, he created a new supporting band featuring Workman, Church, guitarist Gary Pihl, and drummer Chuck Ruff. This lineup played on Hagar's most popular solo album, 1981's platinum Standing Hampton, plus 1982's gold Three Lock Box with only one member missing -- drummer Ruff was replaced by David Lauser. After Three Lock Box and its number 13 hit single "Your Love Is Driving Me Crazy," Hagar played several shows with guitarist Neal Schon, bassist Kenny Aaronson, and drummer Mike Shrieve; the group recorded a live album under the name Hagar Schon Aaronson Shrieve (HSAS), as well as a studio version of Procol Harum's "A Whiter Shade of Pale." His 1984 album VOA contained the hit single "I Can't Drive 55," which peaked at number 26.

In 1985, Hagar replaced David Lee Roth in Van Halen; his first album with the group was 1986's 5150. Hagar released his last solo album in 1987; the record was coined I Never Said Goodbye in an MTV contest. Hagar stayed with Van Halen through the remainder of the '80s and half of the '90s. During that time, the band had four other multi-platinum albums -- OU812 (1988), For Unlawful Carnal Knowledge (1991), Live: Right Here, Right Now (1993), Balance (1995) -- before tensions began to surface between Hagar and the rest of the band.

In the summer of 1996, Hagar either quit Van Halen or was fired; the band had Roth return to sing two tracks on Best of Van Halen, Vol. 1 before hiring former Extreme vocalist Gary Cherone as Hagar's replacement. The entire incident became a media sensation, ensuring that Hagar's 1997 solo album Marching to Mars -- his first in ten years -- would be greeted with much media-generated fanfare. It sold surprisingly well, peaking in the Top 20 and re-establishing Hagar as a viable solo act. With a backing band called the Waboritas in tow (consisting of guitarist Vic Johnson, keyboardist Jesse Harms, bassist Mona, and drummer David Lauser), Hagar followed the success with Red Voodoo two years later; it too sold very respectably on the strength of the single "Mas Tequila," just missing the Top 20.

Hagar's resurgence continued with 2000's Ten 13. Not 4 Sale arrived in 2003, followed by his first live album in 20 years, Live: Hallelujah. Livin' It Up! was released in 2006, while VOA was reissued the following year in deluxe format, courtesy of the newly launched American Beat Records. In 2008 Hagar released Cosmic Universal Fashion, his 11th solo album and first for Roadrunner imprint Loud & Proud Records. That same year, he debuted a new band, Chickenfoot, after jamming with Van Halen bassist Michael Anthony, guitarist Joe Satriani, and Red Hot Chili Peppers drummer Chad Smith. The supergroup released its self-titled debut album in 2009.