Diamond began his music industry career by heading up MRC Music (the publishing arm of Mercury Records). He scored his first #1 record with "This Girl Is A Woman Now," as recorded by Gary Puckett and the Union Gap. He followed up with Steam's #1 "Na, Na, Hey, Hey" (on which he is also was one of the background singers) and then the Supremes' and Temptations' smash duo of "I'm Gonna Make You Love Me." During this period he worked very closely with songwriters: James Taylor, Laura Nyro, Harry Nilson, Van Morrison, The Bee Gees, Rod Stewart, Barry Manilow, and David Bowie.