From the 1960s, Mansfield was associated with an array of notable performers including The Beatles, The Beach Boys, Waylon Jennings, James Taylor, Roy Orbison, Don Ho, the Imperials, Tompall Glaser, Harry Nilsson, Glen Campbell, Buck Owens, Lou Rawls, Andy Williams, The Flying Burrito Brothers, Eric Burdon, Badfinger, Jackie Lomax, The Four Freshmen, Judy Garland, Dolly Parton, David Cassidy, Nick Gilder, Claudine Longet, and Jessi Colter.
At the same time, Mansfield sang with a folk group called The Town Criers and opened a nightclub in San Diego's suburb of La Mesa.
Armed with his marketing degree and a borrowed suit, he was interviewed and then hired in January 1965 as the company's District Promotion Manager West Coast, making him one of the youngest executives with the firm.
Mansfield joined his four new bosses setting up the worldwide launch of Apple Records and the U.S. management of subsequent projects such as The Beatles (aka The White Album), Yellow Submarine, Abbey Road, Let It Be and Hey Jude.
In addition to the Beatles, Mansfield looked after the careers of Apple artists such as James Taylor, Mary Hopkin, Badfinger and Jackie Lomax.
Mansfield came up with the solution by bringing an advance copy of the two songs from the UK to America and playing them to a few trusted radio station managers, who were unanimous in their decision that “Hey Jude” was the hit.
When the Apple empire began to crumble, Mansfield turned down an offer by businessman Allen Klein to stay despite the promise of his salary being tripled.
Mansfield's tenure with Barnaby lasted two years (1971–73) chiefly because he wanted to take the label heavy into the emerging contemporary country market, which evolved into the exciting “Outlaw” movement.
He set up Hometown Productions Inc. and went on to produce the acts that he wanted to bring to Barnaby Records – Waylon Jennings, Willie Nelson, Jessi Colter, Tompall Glaser and other cutting-edge and Outlaw country artists.
Mansfield's five-year producer tenure with the Outlaws started in 1973 with the hit single "We Had It All" from the classic Waylon Jennings album, Honky Tonk Heroes.
A series of Top Ten albums and singles produced by Mansfield with both artists found a place on the charts and playlists in country and pop categories.
Published by Broadman & Holman, the work had three printings and was the only book ever approved by the Beatles (Yoko Ono on John Lennon's behalf) outside their own Anthology.
The White Book has been endorsed by many Apple/Beatles related people including The Rolling Stones' Andrew Loog Oldham, Peter Asher, Alan Parsons, Robin Leach, BeatlesandBeyond Radio show presenter Pete Dicks (who worked on the promotional video for the book) and former Apple President Jack Oliver.
Stumbling on Open Ground, his fourth, most heavily endorsed and insightful book is also a Thomas Nelson Publishing release (January 15, 2013), and reaches a whole new audience for Ken’s innovative writing approach.