Signing up for the SHIELD Academy shortly after Nick Fury joined SHIELD as director, the young and eager Sitwell saw his career take a major turn when he was discovered and mentored in part by Nick Fury.

Despite his nebbish appearance and mannerism, Sitwell in fact graduated a valedictorian of the SHIELD Academy class of 1966. He ranked first in his class in airborne jump school and underwater maneuvers and proved to be a crack shot. Patriotic and unwaveringly loyal to the agency, he had almost perfect recall of the SHIELD manual and followed it to the letter, often even having to remind Fury of the rules and regulations.

Perhaps his most infamous contribution to the agency was his catch phrase, "Don't Yield, Back SHIELD", yelled frequently in battle by him, an obvious homage to the legendary "Wahoo!", battle cry of the Howling Commandos. Sitwell in fact got a chance to fight side by side with the Howlers, reunited for a special mission in Vietnam. Ironically, Jasper's own grandfather Jason, served at the same army base in England where Nick Fury and the Howling Commandos were stationed.

Sitwell earned some respect as interim director of SHIELD while Fury undertook solo missions. Sitwell was later assigned as a body guard to Tony Stark, where Stark was soon besieged with Sitwell's by-the-book need for order, making Stark's career as Iron Man just a little more difficult. During his time at Stark's side Sitwell faced off against such villains as the Grey Gargoyle, the criminal mafia known as the Maggia, A.I.M., Capricorn and Spymaster, who wounded him and left him in a coma.

However Sitwell's greatest challenge was his relationship with the conflicted Maggia operative Whitney Frost, aka Madame Masque. Frost found herself unwittingly falling for the agent as she manipulated him as a tool of the Maggia. Sitwell caught on, but found himself unable to arrest her following a SHIELD assault on the Maggia headquarters. Later things became even more complicated as Frost, in love with both Sitwell and Stark, was seen with Iron Man, driving Sitwell into a rage. The love triangle strained Sitwell and Stark's relationship for a time. Sitwell became unbalanced by his emotions and attacked both Madame Masque and Iron Man. In time however, both men were able to put it behind them.

Moving on from his assignment at Stark Industries, Sitwell proved his worth as a valuable asset to SHIELD and a loyal agent in the eyes of his hero, Nick Fury. His loyalty to the organization however was manipulated by the Deltite LMD. With a power vacuum at the top, and already having served in the post, Sitwell was surprised to be named Fury's successor. Cloned via a hybrid of LMD and human genetics, Sitwell's Deltan LMD was instated as director and used to promote a worldwide hunt for Nick Fury. Eventually the imposter Sitwell met his end aboard the SHIELD satellite. 

The real Sitwell, who had been stored underneath SHIELD Central along with the other missing agents, was brainwashed into a HYDRA agent and placed into suspended animation until 1993 when he was released to battle Fury as part of Von Strucker's final play against Nick Fury.

Following his return to active duty, Sitwell was marginally involved in two dubious SHIELD operations, the Nemesis Contract and the cover-up involving the release of the Omega virus at a Mideast A.I.M. laboratory that tied into the disappearance of agent David Ferrari.

Perhaps thinking he needed a change of pace from fieldwork, Sitwell became an interrogator. This new assignment lead to a renewed camaraderie with with fellow long time SHIELD agent Jimmy Woo, and both work as two of the best interrogators in the agency

.Other Appearances (novels)