Kennerly is the son of O.A. "Tunney" Kennerly, a traveling salesman, and the son of the sheriff of Klamath County, and Joanne Hume Kennerly, the daughter of a railroad engineer. His parents are deceased. He also has three younger sisters, Jane and Chris, the youngest, Anne, is also deceased. His interest in photography started when he was only 12, and his career began in Roseburg, where his first published picture was in the high school newspaper The Orange 'R in 1962. Kennerly graduated from West Linn High School outside of Portland, Oregon, in 1965. While there he worked on the school newspaper The Amplifier and the yearbook, Green and Gold.At 18, right out of high school, he became a staff photographer for the Oregon Journal, and later, after returning from basic and advanced training as a member of the Oregon National Guard, Oregonian, During his early career in Portland he photographed some major personalities, including Miles Davis, Igor Stravinsky, Sen. Robert F. Kennedy, The Rolling Stones, and The Supremes. That encounter with Sen. Kennedy gave him the determination to become a national political photographer.

Kennerly moved to Los Angeles in the fall of 1967 as a staff photographer for UPI. It was there in 1968 that he took some of the last photos of Sen. Robert Kennedy as he declared victory in the California presidential primary at the Ambassador Hotel. Moments later the senator was gunned down by the assassin Sirhan Sirhan. The following year Kennerly moved to New York for UPI where among many other assignments he photographed the Amazing Mets win the 1969 World Series.

In early 1970 Kennerly was transferred to the Washington, D.C. bureau of UPI. At age 23 he took his first ride on Air Force One with President Nixon as a member of the traveling press pool. But Washington wasn't for him, and he felt like he was missing out on the biggest story of his generation, the Vietnam War. Kennerly said, "I felt like that scene in Mr. Roberts where Henry Fonda, an officer on a supply ship, watched the destroyers sail into battle while he was stuck in some South Pacific backwater port."