[3] His career spanned half a century, including starring in the first-run syndicated adventure series Ripcord in the leading role of skydiver Theodore "Ted" McKeever, and as Keith Holden in Lassie.

[12] Regarding his retirement from baseball, sportswriter Furman Bisher was quoted as saying "his future seemed unlimited...I shall always be frustrated by a desire to know how great a star he might have become.

[12] Shortly after his contract was purchased by the Brooklyn organization, he decided not to report to spring training and instead began his career in films.

Early in his acting career, Pennell went to Europe to appear in films, including Old Surehand, a German production based upon a Karl May's novel.

[17] He returned to the United States and made guest appearances in several western television series such as Dick Powell's Zane Grey Theater, Death Valley Days, Have Gun – Will Travel, Wagon Train, Bat Masterson, The Big Valley, The Virginian, Gunsmoke, Bonanza, Rango, Custer, Branded, The Rough Riders, Cimarron City and Tombstone Territory.

[19] His co-star was Ken Curtis, later as Festus Hagen on Gunsmoke, as his inseparable, level-headed older mentor and best buddy James (Jim) Buckley.

[19] More television guest appearances followed on The Outer Limits, Thriller, The Millionaire, The West Point Story, Wire Service, The Case of the Dangerous Robin, Steve Canyon, Sea Hunt, The Aquanauts, The Everglades, Adventures in Paradise, Dragnet, and Suspense Theater.

As Dash Riprock on The Beverly Hillbillies, he appeared in ten episodes as a film star courting Elly May Clampett (Donna Douglas).

[20] After The Beverly Hillbillies, Pennell guest-starred in other television series like Blue Light, My Friend Tony, Mayberry, R.F.D., Family Affair, Land of the Giants, Bracken's World, BJ and the Bear and Salvage 1.

He made guest-starring appearances in various shows, including Mannix, Longstreet, Hunter, Banacek, Mission: Impossible, The Streets of San Francisco, McMillan and Wife, Magnum, P.I., The Rookies, Little House on the Prairie, Owen Marshall: Counselor at Law, O'Hara, U.S. Treasury, Run, Joe, Run, Apple's Way, Silk Stalkings, Diagnosis Murder, Quantum Leap and Firefly and soap operas such as General Hospital and The Young and the Restless.

Pennell also appeared in the big budget World War II film Midway (1976), as Captain Cyril Simard, alongside Charlton Heston and Henry Fonda.

[22] Other films include The Fear (1999), Bubba Ho-Tep (2002) starring Ossie Davis, Five Minutes (2002), Last Confession (2005), Seasons of Life (2006) and The Passing (2011).

Pennell's stage work encompassed over 50 plays, including The Poker Game, Desperate Hours, Pieces of Time and Dead Autumn's Soul.