Merrill was born Morris Miller in Brooklyn, New York, to shoe salesman Abraham Miller, originally Milstein, and his wife Lillian, née Balaban, immigrants from Warsaw, Poland. Lillian claimed to have had an operatic and concert career in Poland (a fact denied by her son in his biographies)and encouraged her son to have early voice training: he had a tendency to stutter, which disappeared when singing. Merrill was inspired to pursue professional singing lessons when he saw the baritone Richard Bonelli singing De Luna in a performance of Il Trovatore at the Metropolitan Opera, and paid for them with money earned as a semi-professional pitcher. In his early radio appearances as a crooner he was sometimes billed as Merrill Miller. While singing at bar mitzvahs and weddings and Borscht Belt resorts, he met an agent, Moe Gale, who found him work at Radio City Music Hall and with the NBC Symphony Orchestra, conducted by Arturo Toscanini.

Merrill's 1944 operatic debut was in Verdi's Aida at Newark, New Jersey, with the famous tenor Giovanni Martinelli, then at the end of his long stage career.

Merrill, who had continued his vocal studies under Samuel Margolis made his debut at the Metropolitan Opera in 1945, as Germont in La Traviata. His appearance in the silly Hollywood movie Aaron Slick from Punkin Crick led to conflict with Sir Rudolf Bing and a brief departure from the Met in 1951. Merrill sang many different baritone roles, becoming, after the on-stage death of Leonard Warren in 1960, the Met's principal baritone. He was described by Time as "one of the Met's best baritones". He also continued to perform on radio and television, in nightclubs and recitals. He retired from the Met in 1976.

For many years, he led services, often in Borscht Belt hotels, on Rosh Hashana and Yom Kippur.

He married soprano Roberta Peters in 1952. They parted amicably; he had two children, a son David and a daughter Lizanne, with his second wife, Marilyn, née Machno, a pianist.

He wrote two books of memoirs, Once More From the Beginning (1965) and Between Acts (1976), and co-wrote a novel, The Divas (1978).

Relatively late in his singing career, Merrill also became known for singing "The Star-Spangled Banner" at Yankee Stadium. He first sang the national anthem to open the 1969 baseball season and it became a tradition for the Yankees to bring him back each year on Opening Day and special occasions. He sang at various Old Timer's Days (wearing his own pinstriped Yankee uniform with the number "1 1/2" on the back) and the emotional pre-game ceremony for Thurman Munson at Yankee Stadium on August 3, 1979, the day after the catcher's death in a plane crash.

Robert Merrill liked to play golf and was a member of the Westchester Country Club in Rye, New York, for many years.

Merrill preferred a traditional approach to the song devoid of additional ornamentation; he told Newsday in 2000, "When you sing the anthem, there's a legitimacy to it. I'm extremely bothered by these different interpretations of it."

Merrill received the National Medal of Arts in 1993.

Robert Merrill died at home in New Rochelle, NY, while watching Game 1 of the 2004 World Series between the Boston Red Sox and the Saint Louis Cardinals, after the Yankees had blown a 3-0 lead to the Red Sox in the American League Championship Series. He is interred at the Sharon Gardens Cemetery in Valhalla, NY, which is a subdivision of The Kensico Cemetery. His headstone features an opera curtain that has been drawn open. In keeping with Jewish tradition, small rocks rest on top of the headstone. His epitaph states, "Like a bursting celestial star, He showered his family and the world with love, joy, and beauty. Encore please."