Straight-talking and down-to-earth Texas attitude is what Alphonso MacKenzie (known simply as 'Al' or 'Mac' to his friends) has brought to his espionage career in both the C.I.A. and S.H.I.E.L.D. While serving as the C.I.A. liaison to S.H.I.E.L.D., Mac came to know most of the upper echelon of S.H.I.E.L.D., including Jimmy Woo and Jasper Sitwell. Never too comfortable with all the almost alien technology on display at S.H.I.E.L.D., Mac found it none too disheartening when he was terminated as liaison to S.H.I.E.L.D. by Deltan LMDs posing as Woo and newly appointed director Sitwell. Catching on to the break-ins at S.H.I.E.L.D. installations, Mac ferreted out an agent still not converted to the Deltite plan and learned of the flat brain scans of many of the agents of S.H.I.E.L.D.. Both he and the agent were captured, but Fury and Pierce rescued them. Soon Mac joined Fury on a trail that lead them to the heart of the Deltite Affair and brought an end to the first incarnation of S.H.I.E.L.D.
MacKenzie returned to the C.I.A. and found himself paired professionally and romantically with former S.H.I.E.L.D. agent Contessa Valentina Allegro De Fontaine. While investigating an apparent new offshoot of H.Y.D.R.A., known as the Death's Head Squad, Mac and Val brought Nick Fury into the investigation, uncovering an alien threat and neutralizing it. Convinced that there was still a need for S.H.I.E.L.D., Fury agreed to start a new organization.Among its first new agents were Mac and Val; however their romance grew awkward under Fury's command. Tension clung to the new team as Mac found himself in a strained relationship with his commander and used as a frequent pawn for Val's public jealous stabs at Fury. Despite this, Mac stayed on with the organization using his interrogation skills to break through the conditioning of the recovered agents from the Deltite Affair. Mac also played a role in interrogating the former Super Agents, revealed to be H.Y.D.R.A. moles.
In recent years, Mac has acted as go-between for S.H.I.E.L.D. and the Daily Bugle, offering information to staff writer Ben Urich. In the fallout of the events that have come to be known as Nick Fury's 'Secret War', MacKenzie leaked to Urich information regarding Fury being ousted from the agency. It’s unclear if Mac's leak to the Bugle was done so under orders from someone within (or outside) the agency or for his own personal reasons (perhaps still holding a grudge against Fury after all these years).
Mac is currently retired from S.H.I.E.L.D. and in 2004 he published, UnSHIELDed: An Unauthorized Insider's Look Behind the World's Most Powerful Global Spy Network.
Al Mackenzie appears in the show starting with the second season, portrayed by Henry Simmons. He acts as the team's mechanic and goes by the nickname "Mac."