In 1994, Kiss' Gene Simmons and Paul Stanley hired Thayer to work part-time on their forthcoming book Kisstory, which led to other projects and eventually a full-time role with Kiss. Thayer's work for Stanley and Simmons began by performing such tasks as painting Stanley's house and cleaning out Simmons' gutters.
Thayer managed the 1995 Worldwide Kiss Convention tour and the Kiss MTV Unplugged concert. In preparation for 1996's Kiss Alive/Worldwide Tour, Thayer worked with guitarist Ace Frehley and drummer Peter Criss, to help them relearn their original guitar and drum parts from the 1970s. Thayer worked as producer and editor of Kiss' long form video and film releases including: Kiss, The Second Coming in 1998, New Line Cinema's feature Detroit Rock City in 1998, and Showtime Television's pay-per-view, The Last Kiss in 2000.
By 2002 and with the growing uncertainty of Ace Frehley's involvement in the band, Thayer stood by for a Kiss performance at the 2002 Winter Olympics Closing Ceremony in Salt Lake City to fill-in on lead guitar if necessary. One month later Thayer got the call and donned the Spaceman makeup for the first time, filling in and performing onstage with Kiss at a private concert in Trelawny, Jamaica. Several TV appearances followed in 2002 including ABC's Dick Clark's American Bandstand 50th Anniversary Show and That 70s Show on Fox.