He attended a military academy and joined the United States Navy when he was sixteen. He was barely eighteen when his destroyer assisted in the Normandy Invasion on D-Day (June 6, 1944). After the war he attended St. Lawrence University in Canton, New York, where he graduated with honors in English. He was also president of the student body, a member of the honor society, editor of the college literary magazine, a football player, and the backfield coach of the only undefeated team in the history of the university. He was a campus radio personality who married the queen of his fraternity's ball during his senior year. After graduation he became a radio sportscaster and DJ for several CBS and ABC affiliates while beginning a family that included three sons and one daughter but ultimately this marriage ended in divorce. Several years thereafter, he had a second daughter.
For six years, Slate had a promising career with W. R. Grace and Co. as a public relations executive and travel manager for company president J. Peter Grace. He then joined Grace Steamship Lines and moved with his family to Lima, Peru. There he joined a professional theatre group, became involved with a production of "The Rainmaker" and was awarded the Tiahuanacothe, the Peruvian equivalent of the Tony Award, for his portrayal of the character Starbuck. After a year of training, he left W. R. Grace to pursue a theatrical career.
Slate co-starred with Ron Ely in the 1960-1961 Ivan Tors series The Aquanauts,:53-54 which was renamed Malibu Run:647 halfway during its brief run on CBS. The series could not compete successfully in the same time slot as NBC's durable western Wagon Train. He guest-starred in nearly one hundred television shows and appeared in twenty feature films. Among his many television appearances were two roles in the courtroom drama series Perry Mason, both times as Perry's client: In season 3, 1960, he played Bob Lansing in the episode, "The Case of the Ominous Outcast", and in season 5, 1962, he played Philip Andrews in "The Case of the Captain's Coins."
He guest-starred in the 1959-1960 syndicated western series, Pony Express, starring Grant Sullivan.
In 1963, Slate was cast as Mark Novak in the episode "The Loner" of the NBC modern western series, Empire, set on a ranch in New Mexico. In the storyline he becomes involved in a deadly boxing match with series character Tal Garrett (Ryan O'Neal). Also in 1963, he co-starred in an episode of the second season of Combat! called "Off Limits," produced and directed Robert Altman.
Slate played Hank in the NBC comedy Accidental Family in 1967-1968.
From 1979 to 1987, Slate portrayed Chuck Wilson on the ABC daytime soap opera One Life to Live. For a short time, from April to October 1985, while Slate was not on One Life to Live, he portrayed the character of Locke Walls on the CBS daytime drama (soap opera) Guiding Light. Slate appeared nine times on CBS's Gunsmoke. He also guest-starred three times on The Alfred Hitchcock Hour on CBS and then NBC, on CBS's Mission: Impossible, ABC's Bewitched, and NBC's My Name Is Earl.
Slate's acting career included major roles in four outlaw biker films in the late 1960s: The Born Losers (1967), The Mini-Skirt Mob (1968), Hell's Belles (1969) and Hell's Angels '69. As the leader of the Born Losers Motorcycle Club in The Born Losers, Slate played a ruthless yet likable character who took on Billy Jack. In Hell's Angels '69 (for which he wrote the screen story) Slate played a man who used the Hells Angels as unwitting dupes in a plan to rob a casino in Las Vegas; several real-life members of the Hell's Angels — including Angels president Ralph "Sonny" Barger, Terry the Tramp and Magoo — had significant speaking roles in the film. Slate broke his leg during filming and never rode a motorcycle again. He also played a role in the western The Sons of Katie Elder starring John Wayne (1965).
Slate was an accomplished country-and-western songwriter and BMI member. He wrote the lyrics to Tex Ritter's top ten song "Just Beyond the Moon" and co-wrote with Greg R Connor the lyrics for "Every Time I Itch (I Wind Up Scratchin' You)" recorded by Glen Campbell on Capitol Records. Slate and Campbell had starred together in the 1969 movie, True Grit.
He was briefly married to the actress Tammy Grimes and was stepfather to actress Amanda Plummer during this time.
In 2004, he attended as a guest at the Western Film Fair in Charlotte, North Carolina, along with Stella Stevens, Andrew Prine and Sonny Shroyer.
On November 19, 2006, Slate died in Los Angeles, California, following surgery for esophageal cancer. He was survived by his partner, Joan Benedict-Steiger, two sons, and two daughters.That Kind of Woman (1959) - Sailor (uncredited)
North by Northwest (1959) - Policeman at Grand Central Station (uncredited)
G.I. Blues (1960) - Turk
Girls! Girls! Girls! (1962) - Wesley Johnson
Wives and Lovers (1963) - Gar Aldrich
I'll Take Sweden (1965) - Erik Carlson
The Sons of Katie Elder (1965) - Ben Latta
The Born Losers (1967) - Daniel 'Danny' Carmody
The Devil's Brigade (1968) - Sgt. Pat O'Neill
The Mini-Skirt Mob (1968) - Lon
The Hooked Generation (1968) - Daisey
Hell's Belles (1969) - Dan
True Grit (1969) - Emmett Quincy
Hell's Angels '69 (1969) - Wes
Drag Racer (1971) - Ron
Curse of the Moon Child (1972)
Centerfold Girls (1974) - Sgt. Garrett
Stranger in Our House (1978) - Tom Bryant
Mr. Horn (1979) - Capt. Emmet Crawford
The Dead Pit (1989) - Dr. Gerald Swan
Voyage of the Heart (1989) - Chairman
Goodnight, Sweet Marilyn (1989) - 'Mesquite'
Dream Machine (1991) - Jack Chamberlain
The Lawnmower Man (1992) - Father Francis McKeen