Wiacek got his start in the mid-1970s as a member of the "Crusty Bunkers" inking collective. For a short time in 1975–1976 he inked backgrounds (over Curt Swan's pencils) on Superman for DC Comics. He moved on to regular inking work for DC, and then in 1978 began a long association with Marvel.
Wiacek has inked over such pencilers as Carmine Infantino on Star Wars, John Romita Jr. on The Uncanny X-Men, June Brigman on Power Pack, John Byrne on Sensational She-Hulk, and Walter Simonson on X-Factor and Orion and George Pérez Brave and the Bold, volume 2.
He has also worked with creators such as Bob Budiansky, Colleen Doran, Ron Garney, Mike Grell, Michael Netzer (Nasser), Kevin Nowlan, Don Perlin, Bill Sienkiewicz, and Barry Windsor-Smith.
Over the years, Wiacek has also inked comics published by Dark Horse and Valiant.
Wiacek taught first-year students at the Joe Kubert School of Cartoon and Graphic Art for a brief period in the early 1990s.