Mattern moved to New York City as a young adult in 1971 with ambitions of becoming a fashion designer, briefly attending Fashion Institute of Technology (FIT) and working as a hairdresser for a while. Mattern soon became fascinated with NYC nightlife especially the work of DJ Larry Levan at the Paradise Garage.
After adopting the pseudonym "Junior Vasquez", he began his career in music production in the 1980s. Vasquez worked with DJ and pop remixer/producer
Shep Pettibone, and together they co-produced and edited numerous singles from artists like Madonna, Whitney Houston, Janet Jackson, MC Hammer, Prince, Pet Shop Boys and others. Vasquez landed a brief residency at Club Bassline, where he began to make a name for himself as a live DJ.
Many of Junior Vasquez's earliest underground hits were released under the name "Ellis D", including the gay house track, "Work This Pussy". Vasquez also released the tracks "Just Like a Queen", "My Lolleata", "It's Scratched", and "Took My Love Away".
From 1989 to 1995, Vasquez was the DJ at the original Sound Factory, a club he co-founded with Richard Grant, which was located at 530 W. 27th St in the Chelsea district. Major record labels began soliciting Vasquez to produce club-friendly remixes for their top singers using his personal beat-driven house style.
Due to a rise in demand from these labels, many singer and musicians like C+C Music Factory, David Morales, Björk, Marilyn Manson, and Madonna were known to frequent his Sunday morning events. Working with many popular artists allowed Vasquez to create his own repertoire of exclusive "Private Collection" of authorized remixes (along with several unofficial remixes) which were not released to the market, and thus made his live sets even more distinctive to his following.
Junior Vasquez also was closely associated at this time with The House of Xtravaganza, a gay ballroom "house" which enjoyed prominence in New York City at this time. In the early 1990s, Vasquez released his single "X", which sampled Danny Xtravaganza say the word "extravaganza!" (the voice sample was taken from Danny Xtravaganza's 1990 single "Love the Life You Live").
After the Sound Factory was closed in February 1995, Vasquez held residencies at the Tunnel (1995-1996), and at ArenA (held at the Palladium) (1996–1997) where his suspended booth was custom designed by designers Dolce & Gabbana. In 1997, he returned to the former site of the original Sound Factory, now called Twilo. Remodeled to include a custom-designed DJ booth for Vasquez's exclusive use and fitted with new cutting-edge audio technology known as "Phazon", Vasquez's residency at Twilo lasted until May 2001, when the venue was closed by order of the city. In 1997, Ian Jenkinson and Inner Rhythm Artists Tribal Gathering enlisted Vasquez for the largest single artist DJ event in UK history at the London Arena. Problems with the venue saw the event split between the two clubs: Ministry of Sound and Cream.
Vasquez held shorter-lived residencies at the various clubs in New York, including Discothèque, Sound Factory (so-named by Richard Grant who retained legal rights to the name) and opened in a new space located in Hell's Kitchen section of Manhattan.
In addition to his work as a DJ and remixer, he has co-produced albums for John Mellencamp, For Real, Lisa Lisa, and Cyndi Lauper as well as individual LP tracks for Madonna, Cher, Wild Orchid, and Kristine W. Vasquez, along with his record label Junior Vasquez Music, is also known for introducing new artists, including many crossover singers, to dance/pop music scene, including Vernessa Mitchell, Jason Walker, Casey Stratton, Quentin Elias, Vivian Green, and Sam Harris. In late October of 2019, Vasquez partnered with his new Manager / Business Partner, Christopher Petrock to reignite the musical magic with their new company Junior Vasquez / House of Ellis, in the studio, on the dancefloor and in other new projects and ventures for 2020 and beyond.