[3] When he was five years old, he moved with his family to the Hainesville section of Sandyston Township, New Jersey, where he attended elementary and secondary school.

[11] Sienkiewicz continued as the artist of the Moon Knight color comics series, starting with the first issue (November 1980).

His eclectic art style helped shed the early perception of Moon Knight as a mere Batman clone.

From this period on, Sienkiewicz's art evolved into a much more expressionistic style, and he began experimenting with paint, collage, and mixed media.

[14] Sienkiewicz produced covers for a range of Marvel titles, including Rom, Dazzler, The Mighty Thor, Return of the Jedi and The Transformers, and drew the comic adaptation of Dune.

Sienkiewicz both wrote and illustrated the 1988 miniseries Stray Toasters, an idiosyncratic work published by Epic Comics about a criminal psychologist investigating a series of murders.

[15] He illustrated the 1986-87 eight-issue Elektra: Assassin limited series[16] and the Daredevil: Love and War graphic novel which were both written by Frank Miller.

[14] In October 2012, Sienkiewicz teamed with fellow artists Klaus Janson and David W. Mack on the eight-issue Marvel mini-series Daredevil: End of Days.

"[22] In June 2014, Sienkiewicz was the guest of honor at ceremony for the 2014 Inkwell Awards at HeroesCon in Charlotte, North Carolina.

[23][24] In April 2022, Sienkiewicz was reported among the more than three dozen comics creators who contributed to Operation USA's benefit anthology book, Comics for Ukraine: Sunflower Seeds, a project spearheaded by IDW Publishing Special Projects Editor Scott Dunbier, whose profits would be donated to relief efforts for Ukrainian refugees resulting from the February 2022 Russian invasion of Ukraine.

In 1989, Sienkiewicz painted the art for the Friendly Dictators card set published by Eclipse Comics which portrayed various foreign leaders such as Mobutu Sese Seko, Ferdinand Marcos, and Anastasio Somoza Debayle.

Additional Sienkiewicz album covers include RZA's Bobby Digital in Stereo (1998), EPMD's Business as Usual (1990), and Kid Cudi's Man on the Moon: The End of Day (2009).