 McDonald was recruited by the rock band the Doobie Brothers in April 1975, initially as a temporary replacement for their lead vocalist Tom Johnston after he had become ill during a national tour, but his work with the band proved so successful that they decided to retain him as a full-time member.As a member of the Doobie Brothers, McDonald sang the lead vocals on some of the band's best-known songs such as "Real Love", "Takin' It to the Streets", "Little Darling (I Need You)", "It Keeps You Runnin'", "Minute by Minute", and "What a Fool Believes" (which became a number-one single in the U.S. and earned him a 1980 Grammy Award for Song of the Year along with co-writer Kenny Loggins).At the same time, McDonald appeared as a session vocalist and keyboardist for various artists, including Christopher Cross, Stephen Bishop, Jack Jones, Bonnie Raitt, the rock band Toto, and Kenny Loggins.