Originally recorded by Captain & Tennille (May 1975).
Hit versions by David Cassidy (UK #11 July 1975), Barry Manilow (US #1 Oct 1975).
Also recorded by Bruce Johnston, composer (1977).
No. Barry Manilow did not ‘write the songs’. ‘I Write the Songs’ was written by Bruce Johnston, of the The Beach Boys, in 1975.
From the wiki: “The original version was recorded by Captain & Tennille, both of whom worked with Johnston in the early 1970s with The Beach Boys. The song appears on their 1975 debut album, Love Will Keep Us Together, but was never released as a single. (Daryl ‘Captain’ Dragon is quoted as saying the pair wanted to release ‘I Write the Songs’ as their debut single but, instead, opted for ‘Love Will Keep Us Together‘.)