Culp fell and hit his head while taking a walk outside his Hollywood Hills home. He was found by a jogger who called 911 and was pronounced dead at Hollywood Presbyterian Medical Center in Los Angeles, said Lt. Bob Binder of the Los Angeles Police Department. An autopsy is pending.
"My mind wants to flow into sadness, but I want to stay above that," Cosby told The Times on Wednesday.
"Those of us who are the firstborn always dream of that imaginary brother or sister who will be their protector, the buffer, the one to take the blows," Cosby said. "I'm a firstborn, and Bob was the answer to my dreams. He was the big brother that all of us wish for."
Longtime friend Hugh Hefner, who was introduced to Culp by Cosby in the 1960s, said he was "absolutely stunned" by the actor's death.
"He was one of my best friends," Hefner told The Times on Wednesday.
Culp was a regular at a weekly gathering of friends at the Playboy Mansion.
"He was very much like he appeared to be," Hefner said. "He's the one who came up with the tongue-in-cheek motto for when the guys got together: 'Gentlemen, gentlemen, be of good cheer, for they are out there and we are in here.' "
In a six-decade career in which he was best known for his work on television, Culp first came to fame as the star of the TV western "Trackdown," which ran on CBS from 1957 to 1959 and featured Culp as Texas Ranger Hoby Gilman.
He later played FBI Agent Bill Maxwell on the 1981-83 ABC series "The Greatest American Hero."
But for TV fans of a certain age, Culp is best remembered for “I Spy.”
The hourlong series, which ran from 1965 to 1968 and was billed as an "adventure-comedy" by NBC, starred Culp as Kelly Robinson and Cosby as Alexander Scott, American secret agents whose cover was that Kelly was a globe-trotting top-seeded tennis player and Scott was his trainer.
The series, which was filmed on location around the world, made history as the first American weekly dramatic series with a black performer in a starring role.
"When I first heard Bill Cosby was the other half of this team, I said, 'Wait a minute!' " Culp told The Times in 1965. "I knew he was a comedian, but could he act? Then I saw him work in our pilot film, and the guy is brilliant.
"We have a rapport never seen on a screen before. It's a kind of Clark Gable-Spencer Tracy relationship. We're an inseparable team, a kind of Damon and Pythias. Bill and I together form what you will root for in the series."
Culp received three consecutive Emmy nominations for his role in "I Spy" and was beat out each time by Cosby.
But Culp, who also received an Emmy nomination for a script he wrote for the series, said he wasn't jealous over Cosby's wins.
"No," he told the Washington Post in 1977, "I was the proudest man around."
In a 1969 Playboy interview, Cosby said that after he and Culp first read for the series, they got together afterward and talked.
At Culp's suggestion, he said, "we agreed to make the relationship between the white character, Kelly Robinson, and the black man, Alexander Scott, a beautiful relationship, so that people could see what it would be like if two cats like that could get along."
Culp appeared in more than two dozen films, including "PT 109" and "Sunday in New York." He also directed the 1972 crime drama "Hickey & Boggs," starring himself and Cosby.
Most notably on the big screen, however, Culp starred in "Bob & Carol & Ted & Alice," writer-director Paul Mazursky's 1969 comedy-drama about two upper-middle-class Los Angeles couples dealing with the idea of sexual freedom.
Culp and Natalie Wood played Bob and Carol; Elliott Gould and Dyan Cannon were Ted and Alice.
Culp, critic Roger Ebert wrote in his review of the film, "is the essence of the sagging, dissipated early-middle-age swinger."
Mazursky, who was shocked and saddened to hear of Culp's death, said Wednesday that Culp was "terrific" in the film.
It was, he said, producer Mike Frankovich's idea to cast Culp as Bob.
"He said, 'What about the guy from 'I Spy'?" recalled Mazursky. "I met with Bob and liked him and gave him the part, and he was just wonderful."
Gould first met Culp when they did the movie.
"There was always something positive between us," Gould told The Times. "He was very generous and very kind and sensitive, and I felt that he was really a loving guy."
Culp, who was involved in civil rights causes in the '60s, also was active in civic causes.
In 2007, he joined real estate agent Aaron Leider in filing a lawsuit against Los Angeles Zoo Director John Lewis and the city to stop construction of a $42-million elephant exhibit and bar the zoo from keeping elephants there, accusing authorities at the facility of withholding medical care from the animals and keeping them cramped in small spaces.
Last year, after temporarily halting construction on the elephant exhibit amid a fierce debate, the City Council voted to proceed with the project as planned.
Culp was born in Oakland on Aug. 16, 1930. A 1947 graduate of Berkeley High School, he studied drama at a number of colleges.
He left for New York before earning a degree and appeared in off-Broadway productions and live TV anthology series such as "Kraft Television Theatre," "Alfred Hitchcock Presents" and "The United States Steel Hour" before landing his role in "Trackdown."
Culp, who was married five times, is survived by his sons Joseph, Joshua, Jason and daughters Rachel and Samantha; and five grandchildren.
Services are pending.
Times staff writers Andrew Blankstein, Valerie J. Nelson and Greg Braxton contributed to this report.