McClintock was an employee in the finance department of Union Allied and an associate of Wilson Fisk. He was killed by one of Fisk's men, which was staged as a suicide to prevent Union Allied's money laundering from being traced back to Fisk's organization.
During his time as responsible for the financial department of Union Allied Construction, McClintock used the company to transfer money for Wilson Fisk, its leader. Karen Page, the secretary to the chief accountant, opened a file showing these money transfers, covered as company pension. When she visited McClintock to tell him about her discovery, he claimed the file was a theoretical model Union Allied was experimenting with. Afterwards, McClintock presumably discovered Page took a copy of the file and planned to discuss legal action with Daniel Fisher, prompting him to alert Fisk.
Page's copy of the file were eventually given to the New York Bulletin, causing Union Allied Construction to disband once the story became public. To ensure McClintock would not talk about his illicit activities, he was killed by Fisk's men, making it look like he overdosed on pills, and made to be the fall guy for Union Allied's money laundering.
When Fisk ordered James Wesley to get John Healy killed, Wesley told him that with the murders they committed lately, including the one on McClintock, they are leaving a trail of bodies that would eventually lead somewhere.
Since the fall of Union Allied Construction and the murder of Daniel Fisher, Page was determined to expose a criminal empire operating in Hell's Kitchen. She brought her case to Ben Urich, an investigative journalist at the New York Bulletin. During one of their meetings, Urich warned Page that McClintock had "overdosed on pills, or some such", which was likely arranged to cover up the company's dealings.