Answer by taking a quote from the following article:

Bernard Mathew Leadon III (pronounced led-un; born July 19, 1947) is an American musician and songwriter, best known as a founding member of the Eagles. Prior to the Eagles, he was a member of three pioneering and highly influential country rock bands: Hearts & Flowers, Dillard & Clark, and the Flying Burrito Brothers. He is a multi-instrumentalist (guitar, banjo, mandolin, steel guitar, dobro) coming from a bluegrass background. He introduced elements of this music to a mainstream audience during his tenure with the Eagles.

Leadon was the last original member to join the Eagles, a band initially formed by guitarist/singer Glenn Frey, drummer/singer Don Henley, and former Poco bassist/singer Randy Meisner. Leadon is often credited with helping shape the band's early country-rock sound, bringing his strong sense of harmony as well as his country, bluegrass and acoustic sensibilities to the group. Instruments he played during his tenure in the band were electric guitar, B-Bender, acoustic guitar, banjo, mandolin, dobro and pedal steel guitar. Upon the release of their debut album, Eagles, the group met with near instantaneous success, due largely to the strength of their hit singles, "Take It Easy", "Peaceful Easy Feeling" and "Witchy Woman" (co-written by Leadon and Henley), all of which highlighted Leadon's multi-instrumental talent on electric guitar, B-Bender, banjo, and harmony vocals. Their follow-up, Desperado, was another strong country-rock venture highlighted by the classics "Tequila Sunrise" and the title track. Leadon had a prominent role on the album, but it was met by surprisingly lukewarm reviews and lackluster sales. As a result, the band attempted to distance itself from the "country rock" label for their third album On the Border. In doing so, Leadon encouraged the group to recruit his old friend, guitarist Don Felder, to the band. The result was the guitar-heavy top ten hit "Already Gone". The album also included "My Man", Leadon's touching tribute to his old bandmate and friend, Gram Parsons, who had died of a drug overdose the year prior at Joshua Tree National Monument in southeastern California. With the wild success of On the Border and its follow-up smash, One of These Nights, tension within the band grew, as Leadon grew increasingly frustrated by the band's direction away from his beloved country and bluegrass and toward album-oriented stadium rock. He famously quit the band in 1975 by pouring a beer over Glenn Frey's head. He later cited a need to get healthy and break the vicious cycle of touring, recording and heavy drug use that was rampant within the band. Upon his departure, Asylum Records released Their Greatest Hits (1971-1975), which highlighted the band's Leadon years and went on to become the biggest-selling album of all time for sales in excess of 42 million units, awarded to the band members by the RIAA. He was replaced by former James Gang guitarist/singer, Joe Walsh. Although it has long been believed that he left because he was dissatisfied with the band moving into rock and roll, Leadon denies it and said in 2013: "That's an oversimplification; it implies that I had no interest in rock or blues or anything but country rock. That's just not the case. I didn't just play Fender Telecaster. I played a Gibson Les Paul and I enjoyed rock & roll. That's evident from the early albums."

When did Leadon join the Eagles?
Upon his departure, Asylum Records released Their Greatest Hits (1971-1975), which highlighted the band's Leadon years