He and his future wife began writing together in 1964 and found success early when a throwaway party tune, “Let’s Go Get Stoned,” (written with a third partner) rose to No. 1 on the R&B charts for Charles in 1966.
The couple then became staff writers for Motown and churned out a series of hits in the late 1960s for Gaye and his singing partner, Tammi Terrell, that included the rousing “Ain’t No Mountain High Enough,” “Your Precious Love” and “Ain’t Nothing Like the Real Thing.”
Early in their career, Mr. Ashford primarily wrote lyrics while Simpson composed the music at a piano.
“We still don’t have a formula,” Mr. Ashford told The Washington Post in 1977. “I might think of a line or two. Valerie might hear me singing and try to catch up on the piano. But then I might hear her playing and come with an idea.”
The couple also wrote for Gladys Knight & the Pips, Smokey Robinson, Aretha Franklin, The Marvelettes, Teddy Pendergrass and Chaka Khan.
“They had magic, and that’s what creates those wonderful hits, that magic,” Verdine White of Earth, Wind and Fire told the Associated Press. “Without those songs, those artists wouldn’t have been able to go to the next level.”
Ross and the Supremes scored a hit with “Ain’t No Mountain” in 1968. Two years later, she recorded a solo version that became her first No. 1 hit.
Mr. Ashford and Simpson wrote and produced all but one of the songs on Ross’s debut solo album, “Diana Ross” (1970), including the enduring hit “Reach Out and Touch (Somebody’s Hand).”
“We had been friends a long time,” Ross said in 1980, “and I knew what kind of songwriters they were. . . . Nick and Valerie have been my good-luck charm.”
Motown founder Berry Gordy objected when the songwriting duo wanted to branch out and pursue a performing career. As a result, they left Motown in 1973 and found considerable success as an R&B-disco crossover group well into the 1980s. Their 1984 hit “Solid (as a Rock)” reached No. 1 on the R&B charts and No. 12 on the Billboard pop chart.
During the 2008 presidential campaign, the Ashford & Simpson hit found a second life when a “Saturday Night Live” skit reconfigured it as “Solid as Barack.”
Nickolas Ashford was born in Fairfield, S.C., on May 4, 1941. (Many reference sources state that he was born one year later, but public records confirm the year of his birth as 1941.)
He grew up in Willow Run, Mich., and got his musical start in a Baptist church. He briefly attended Eastern Michigan University before moving to New York to pursue a career as a dancer.
He slept on park benches and had been homeless for several months when he met Simpson at a Harlem church. They soon teamed up to write gospel tunes before turning to love songs.
“I get bored when I’m not writing about love,” Mr. Ashford told the New York Times in 1985. “Politics or social commentary don’t inspire me. Love lifts me up.”
The couple married in 1974. In addition to his wife, of New York, survivors include two daughters, his mother and two brothers.
In 2005, Mr. Ashford told the Ann Arbor (Mich.) News that “Ain’t No Mountain” was inspired by his sense of awe in New York City: “I was walking in Central Park wondering how I was going to make it in the big city. It was so big with so many people. I looked up and the tall buildings were mountains. ‘Ain’t No Mountain High Enough’ is about being inspired by courage to make it.”