He started his first band at secondary school with guitarists Karel Jansen and Roelof Egmond and singer Anneke Grönloh.

In 1977, Koelewijn released his debut solo album Het Beste in Mij Is Niet Goed Genoeg Voor Jou (The Best I Can Give Is Still Unworthy Of You) which spawned two singles; the Rockets-like "Je Wordt Ouder Papa" ("You're Growing Older Papa"; a jocular comment by Golden Earring guitarist George Kooymans) and "KL 204 (Als Ik God Was)", originally intended as the lead single and reissued in 1990.

In 1981, Koelewijn recruited musicians from the band Bunny for a new Rockets line-up that lasted till 1997; their first release was the live-album Peter Live of which "Kom Van Dat Dak Af" and "Klap Maar in Je Handen" hit the charts.

Early 1985, Koelewijn had a top 40 hit with "Sprong in Het Duister" ("Leap in the Dark"), the lead single of his otherwise unsuccessful second solo album.

In 1987, he released "De Loop Van Een Geweer" ("Barrel of a Gun") which was loosely based on The Commodores' "Nightshift".

In 1995, Koelewijn fronted the novelty group Gompie; their version of Smokie's "Living Next Door to Alice" topped the charts.

He opened the show by performing "Kom Van Dat Dak Af" as a duet with singer-songwriter Boudewijn de Groot.