Complex sentence: "City of Angels" was written by lead vocalist Jared Leto, who also produced the song with Steve Lillywhite, having previously worked with Thirty Seconds to Mars on the production of the band's third studio "This Is War" (2009).
Split into simpler sentences: City of Angels was written by lead vocalist Jared Leto, who also produced the song with Steve Lillywhite. The producer had previously worked with Thirty Seconds to Mars on the production of the band's third studio "This Is War" (2009).