Greenfield also co-wrote the theme songs to numerous 1960s TV series, including Gidget, Bewitched, The Flying Nun and Hazel.

In 2005, "(Is This the Way to) Amarillo", a song Greenfield had written with Sedaka in the early 1970s, reached #1 on the UK charts in the original 1971 version by Tony Christie.

[6] In 1958, Greenfield and Sedaka signed to Al Nevins and Don Kirshner's Aldon Music as songwriters, which had offices at 1650 Broadway in New York.

Carol", "Stairway to Heaven", "Calendar Girl", "Little Devil", "Happy Birthday Sweet Sixteen", "Next Door to an Angel" and the chart-topping "Breaking Up Is Hard to Do".

[9] As Sedaka's promotional and touring commitments began taking up more and more of his time, Kirshner encouraged Greenfield to collaborate with other Aldon writers.

Beginning in 1960, Greenfield began a regular collaboration with Jack Keller; they would write songs together every Monday and Wednesday for six straight years.

[12] Greenfield also collaborated with other Aldon songwriters, including Helen Miller, with whom he co-wrote "Foolish Little Girl" (the Shirelles' final Top Ten hit), "It Hurts to Be in Love", originally intended for Neil Sedaka but ultimately recorded by Gene Pitney, as well as a new theme for the TV series Hazel for its fourth season.

As well, Greenfield's one and only collaboration with Aldon songwriter Carole King resulted in "Crying in the Rain", a top ten hit for the Everly Brothers in 1962.

After Sedaka's singing career cooled in 1963, they kept writing hits for other artists, including the 5th Dimension's and Tom Jones' "Puppet Man".

Greenfield was openly gay[17] and was in a domestic partnership with cabaret singer Tory Damon (September 29, 1939 – March 30, 1986) from the early 1960s until his death; the two lived together in an apartment on East 63rd Street in Manhattan before moving to California in 1966.