Matlock attended Saint Martin's School of Art until 1974. He was the original bass player of the Sex Pistols, having been introduced to guitarist Steve Jones and drummer Paul Cook while working in SEX, Malcolm McLaren's clothing boutique in London. He is credited as co-writer on 10 of the 12 songs appearing on the album Never Mind the Bollocks, Here's the Sex Pistols. However, his overall contribution to these songs has been disputed: Jones said in a 2011 interview he was "tired of Matlock's claims that he had co-written some of the punk icon's biggest tunes", stating that he himself had written as many songs as Matlock, whilst Matlock himself notes in his book that the band only wrote two songs after his departure. According to a 2014 interview, he played a big role in writing the songs that appeared on the album and Cook has stated that Matlock wrote most of the songs appearing on the album. Additionally, whilst Jones has insisted that Matlock disliked many of Johnny Rotten's controversial lyrics, Matlock has said that he had no issue with them.
In late 2011, the International Swingers, a band comprising Matlock, Clem Burke, James Stevenson and Gary Twinn, was formed. The band, based in Los Angeles, toured Australia and continues to perform occasionally.