Bobby Hart, the songwriter who helped define the sound of 1960s pop with the Monkees’ most enduring hits, has died at the age of 86.

Hart, who had been in poor health since breaking his hip last year, died at his home in Los Angeles, his friend and co-author Glenn Ballantyne confirmed to the Associated Press.

As one half of the songwriting duo Boyce and Hart, alongside collaborator Tommy Boyce, Hart shaped the sound of a band that began as a TV project but quickly became one of the most iconic groups of the decade. Together they penned six tracks for the Monkees’ smash 1966 debut album and produced many of their early hits, leaving a lasting mark on pop music.

Their most famous success came with “Last Train to Clarksville,” the Monkees’ first No. 1 single, and “I’m Not Your Steppin’ Stone,” both of which cemented their reputation as hitmakers. Hart also co-wrote the Monkees’ theme song, which introduced the group with the line: “Hey, hey, we’re the Monkees.”

“Boyce and Hart were instrumental in creating the unique Monkee sound we all know and love,” Monkees drummer and vocalist Micky Dolenz wrote in the foreword to Hart’s 2015 memoir.

Outside of their songwriting work, Boyce and Hart found fame in their own right. They released records as a performing duo, campaigned for Robert F. Kennedy during his 1968 presidential run, and even appeared in sitcoms at the height of their popularity.

Tommy Boyce died in 1994, but the pair’s legacy was celebrated in the 2014 documentary The Guys Who Wrote ’Em, which revisited their outsized influence on pop culture.

Hart’s personal life was marked by two marriages. He is survived by his second wife, Mary Ann Hart, and two children from his first marriage.

From the bubblegum joy of the Monkees’ theme to the urgency of “Last Train to Clarksville,” Hart’s work defined the sound of a generation. His songs captured the innocence and energy of the 1960s, and their enduring popularity ensured that his name would always be linked to one of pop’s most beloved groups.