William Nack in 2015. He was best known for his writings on the legendary racehorse Secretariat. (Timothy D. Easley/AP)

William Nack, a Sports Illustrated journalist who became a lyrical, sharp-eyed chronicler of horse racing — notably the Triple Crown thoroughbred Secretariat — and who also wrote haunting profiles of disabled football players, late-career racecar drivers and retired boxers, died April 13 at his home in Washington. He was 77.

The cause was cancer, said his wife, Carolyne Starek.

In a 23-year career at Sports Illustrated, and earlier as a political reporter and sports columnist at Newsday, Mr. Nack established himself as a meticulous craftsman. At the magazine, his stories ranged from a 6,000-word account of Jackie Robinson’s 1947 breakthrough on the baseball diamond to a quixotic tale of his two-year search for chess prodigy Bobby Fischer.

“To find him, to see him, had become a kind of crazy and delirious obsession, the kind of insanity that has hounded other men in search of, say, the Loch Ness monster,” Mr. Nack wrote in his 1985 article on Fischer, who shunned publicity after winning a world championship in the 1970s. Mr. Nack disguised himself as a bum to avoid scaring off the chess grandmaster and tracked him to the main branch of the Los Angeles Public Library.

His chief subject was horse racing. He had fallen under its spell as a child, when a horse named Swaps walked to the rail and licked the back of his hand at the old Washington Park Race Track, and he set about memorizing every Kentucky Derby winner, beginning with Aristides in 1875. Later, as a teenager, he vomited from excitement while listening to a dramatic radio call in the Chicago suburbs.

Mr. Nack was named Newsday’s horse-racing correspondent after he mounted a desk at a 1971 Christmas party and, one or three eggnogs deep, recited the names of all 96 winning horses. (At other celebrations, he was known to recite poems by Yeats or Eliot, or the last page of “The Great Gatsby” — in English and in Spanish.)

Mr. Nack in 2010, at the premiere of “Secretariat.” He served as a consultant for the film. (Alberto E. Rodriguez/Getty Images for Disney)

He soon gained national renown for his coverage of a young chestnut colt named Secretariat, who in 1973 swept the Kentucky Derby, Preakness Stakes and Belmont Stakes, becoming the first horse to win the Triple Crown since Citation in 1948. Mr. Nack served as the horse’s Boswell, following him from race to race and reporting on his many quirks and idiosyncrasies.

The horse was “bothered by a pigeon feather at Pimlico on the eve of the Preakness,” Mr. Nack wrote (the writer kept the feather in his wallet for decades), and hours before the Belmont Stakes “burst from the barn like a stud horse going to the breeding shed,” and then walked “around the outdoor ring on his hind legs, pawing at the sky.”

Mr. Nack’s reporting on Secretariat culminated in a 1975 book, “Big Red of Meadow Stable,” that “Seabiscuit” author Laura Hillenbrand once hailed as “the gold standard of horse books.” The work was excerpted by Sports Illustrated, which Mr. Nack joined as a staff writer in 1978, and republished in 2002 as “Secretariat: The Making of a Champion.” It later served as the source material for Disney’s 2010 movie “Secretariat.”

Mr. Nack also examined the seamier side of horse racing, covering the escalating use of painkillers to keep old horses running. His 1993 feature on the subject began with a painfully detailed description of a filly named So Sly, whose left foreleg snapped in half during a race at Pimlico.

Mr. Nack was at the racetrack that day almost by chance, his former editor Sandy Padwe recalled in a telephone interview, and ran from the press box to the track to watch as a vet injected the horse with “100 cc’s of a purple solution.”

The filly was euthanized, he reported, just as “two men, standing behind the rail, raised a turquoise screen to block the view of distant spectators and so protect them from their feelings.”

Mr. Nack “had the best eye and ear that I think I’ve ever seen,” said Padwe, a retired journalism professor at Columbia University. “He took in scenes, he took in details and he knew how to meld it. He didn’t just get details for the sake of having them. He made every one of them seem to work.”

William Louis Nack was born in Chicago on Feb. 2, 1941, and raised in nearby Skokie, Ill. His father was an electrical engineer, and his mother a ballerina. He began working with horses as a child, cleaning a neighbor’s stalls, and eventually became a groom at Arlington Park.

When a sport-photographer uncle took him to the Kentucky Derby in 1958, Mr. Nack developed an interest in journalism, enthralled by the idea that he could “make a living up the road” by writing about the sport and its animals. He became sports editor of the student newspaper at the University of Illinois, where one year later he succeeded his friend Roger Ebert as editor in chief.

After graduating in 1966, Mr. Nack enlisted in the Army and was deployed to Vietnam, where he wrote for military publications and drowned out the late-night sounds of bomb blasts with recordings of major horse races, mailed across the sea by his mother.

He joined Newsday in 1968 and later wrote for magazines including ESPN and GQ. Many of his stories were collected in the 2003 book “My Turf: Horses, Boxers, Blood Money, and the Sporting Life.” It was followed by “Ruffian: A Race Track Romance” (2007), about a celebrated racehorse that broke down and was euthanized in 1975.

Mr. Nack received the PEN/ESPN Lifetime Achievement Award For Literary Sports Writing in 2017.

His first marriage, to Mary Elizabeth Scott, ended in divorce. Survivors include his wife of 14 years, Carolyne Starek; four children from his earlier marriage, Emily Nack of Arlington, Va., Rachel Nack of Pahoa, Hawaii, Amy Nack of Lafayette, Colo., and William Nack of Boulder, Colo.; a sister; and six grandchildren.

At Sports Illustrated, Mr. Nack wrote features on racecar driver A.J. Foyt, Joe Frazier’s out-of-the-ring duel with Muhammad Ali, and “the wincing, hobbling wounded” — retired NFL players who suffered pain and disability decades after leaving the game.

Among his most acclaimed pieces was “Pure Heart” (1990), in which he chronicled the life and 1989 death of Secretariat, whom he had previously visited each year during Derby season.

He knew all of the horse’s stories, he wrote, “knew them well, had crushed and rolled them in my hand until their quaint musk lay in the saddle of my palm. Knew them as I knew the stories of my children. Knew them as I knew the stories of my own life.”

“Horses,” he continued, “have a way of getting inside you, and so it was that Secretariat became like a fifth child in our house, the older boy who was off at school and never around but who was as loved and true a part of the family as Muffin, our shaggy, epileptic dog.”