Ken Howard holds the award for best supporting actor in a miniseries or movie for "Grey Gardens" in 2009. (Lucy Nicholson/Reuters)

Ken Howard, a Tony Award-winning actor who starred as a basketball coach in the TV series “The White Shadow” and served as president of the actors’ union SAG-AFTRA, died March 23. He was 71.

The union announced the death but did not disclose the cause.

Mr. Howard had been a Broadway stalwart since the late 1960s. In Robert Marasco’s Grand Guignol thriller set in a Catholic boarding school, “Child’s Play” (1970), he won a Tony Award for best featured actor, portraying a young, all-American gym coach.

He played Thomas Jefferson in the Broadway musical “1776” (1969), and he reprised the role for the 1972 film. He later appeared on Broadway in 1975 in Alan Ayckbourn’s trilogy “The Norman Conquests” — as Tom opposite Richard Benjamin’s Norman — and in the long-running Neil Simon romp “Rumors” (1988).

Mr. Howard became a staple on television. In the CBS series “The White Shadow,” which aired from 1978 to 1981, he starred as a white coach to an urban high school basketball team. The part drew on the personal history of the 6-feet-6-inch-tall actor, who played basketball growing up on Long Island in New York and at Amherst College in Massachusetts.

He starred opposite Blythe Danner in “Adam’s Rib” on ABC in the 1970s and appeared as the chipper Kabletown boss Hank Hooper on NBC’s “30 Rock” some 40 years later.

In early seasons of NBC’s “Crossing Jordan,” which premiered in 2001, he played the father of star Jill Hennessy, a retired police detective who gave behind-the-scenes advice to his daughter, a crime-solving forensic pathologist. He starred opposite Jimmy Smits in the 2007 CBS drama “Cane” and won an Emmy for his performance as Phelan Beale, the husband of Jessica Lange’s Big Edie, in HBO’s “Grey Gardens” in 2009.

Mr. Howard’s film debut was a co-star role opposite Liza Minnelli in “Tell Me That You Love Me, Junie Moon” (1970), about three outcasts who set up house together, and his later films included “In Her Shoes” (2005) and “Michael Clayton” (2007).

Kenneth Joseph Howard Jr. was born in El Centro, Calif., on March 28, 1944. He grew up in Manhasset, on Long Island. He graduated in 1966 from Amherst and attended the Yale School of Drama before leaving because of his professional commitments on Broadway.

Mr. Howard was elected president of the Screen Actors Guild in 2009 and was a catalyst for its 2012 merger with the American Federation of Television and Radio Artists union. Combined, the groups represent 160,000 actors, broadcasters and recording artists.

Mr. Howard was the first president of SAG-AFTRA, and was reelected to the post last year.

His marriages to actress Louise Sorel and Margo Lederer (the daughter of advice columnist Ann Landers) ended in divorce. Survivors include his wife of 25 years, stuntwoman Linda Fetters Howard; and three stepchildren.